From Smite to Song does exactly what it says on the tin: you can trigger Inspire Courage with smite attempts. Oh, look at that, the paladin just went up a tier. Inspire Courage just fits with the paladin-as-leader, doesn’t it?

Note that the build does employ Harmonious Knight, but does not employ first substitution level (which, confusingly,also grants Inspire Courage - more frequently, but locked to a mere +1 bonus - in place of Detect Evil). Smite to Song is the better effect, and keeping Detect Evil helps remind you that you're still a paladin first and foremost.

There is an argument that, since From Smite To Song isn't actually "Bardic Music", your Inspire Courage won't work with bardic-music-enhancing effects, of which there are several in this build. The counterargument is that none of the boosts in this build (except for Song of the Heart) actually require "Bardic Music". They don't alter Bardic Music so much as explicitly alter the effects of the Inspire Courage bardic music ability, which is the exact ability you're using, except you're using it through From Smite to Song rather than through Bardic Music. The same logic applies to Inspire Greatness from the Harmonious Knight, for exactly the same reason. If you're in an environment where this distinction matters and you can't learn Song of the Heart at 9th level, exchange it for Extra Smiting or another combat feat, or for Leadership if it's available.

The Dungeonscape variant replaces your mount – bear with me here. Normally, the mount is nothing more than a gimmick, and Wizards openly admitted that no matter how much you want to ride it where you like, the battlefield doesn’t always let you. This variant lets you summon spirits to support your team instead. The Healing spirit comes with twice your Lay on Hands capacity and basically lets your allies heal themselves with standard actions (think of it as a location-dependent but much higher capacity potion), so it’s best used out of combat, but since you have 20 Paladin levels in this build, the Lay on Hands capacity will be substantial.

Note that if you'd like, you can grab the second Mystic Fire Knight substitution level here - it has no drawback. However, since it augments a smite attack (and the effect only lasts for a single round!), and you're spending your smites on Bardic Music, its only real advantage to you most of the time is the slightly expanded list of class skills.

At this point, you’re a paladin splashing roughly equal parts of bard and wizard on the side, but without any mechanical multiclassing or alignment concerns. (Somehow, this is rather appropriate for an illumian.) Let me explain.

Thanks to Mystic Fire Knight and Aeshkrau, you have plenty of spell slots for a non-primary caster (slightly behind the bard), and you’ll only get more as you continue to improve your Strength (which you'd do anyway as a warrior). Thanks to Sword of the Arcane Order, those slots can hold wizard spells, such as Enlarge Person (or, later on, Polymorph), and you have a higher-than-usual paladin caster level (6th, at this point) to boot. Put the three together and you’re clearly using a kind of magic that normal paladins wouldn’t even think possible. You also just hit every warrior’s first major breakpoint, base attack +6, so cheer for multiple attacks.

There’s some ambiguity in the Sword of the Arcane Order feat though. It’s not entirely clear if those spells are “paladin” spells or “wizard” spells, even though they clearly use the former’s caster level and, apart from ability scores, appear to use the same overall mechanics. As a result, we assume here that they’re cast as paladin spells. Among other things, this means they avoid arcane spell failure – bring on the heavy metal.

Oh, and why the lame Inspire Competence? First, Remove Disease is even lamer than Inspire Competence, so you're not losing anything by taking it. Second, it's a prerequisite for your next feat.

The combat spirit provides sacred bonuses to attack and damage, which stack with your Inspire Courage abilities. And both the duration and the size of the bonus scales by paladin level – good thing we’re going the full 20 here. Adding in the third-level wizard spells certainly doesn’t hurt either – Haste alone will make a supersonic man out of you.

This feat is perfectly fine for paladin casting: it lets you cast most of your spells as swift actions. In a build like THIS, with wizard spells that you cast as paladin spells, and enough slots to have passable magic stamina? This is positively amazing. Your CL is 10, not too far behind the full casters and high enough to hit most caps or breakpoints in 3rd level spells, so they'll  be effective, too.

Oh, and due to the way Inspire Greatness is worded, you don't just use it twice per day - each use now applies to two targets.

Aww hells yeah! Setting aside the mechanical awesomeness that is this feat, you also have a great thematic tie-in between it and your Illumian (you know, the "Words Made Flesh") heritage, and everyone expects the paladin to be an exemplar for Good. All things considered, a perfect match.

Mechanically, your Inspire Courage hit +4 last level, and here it hits an impossibly good +8 once you use the Words. (If you use the Badge of Valor, you’re Inspiring at +10. The Vest of Legends gets that up to +12, and you can afford that by yourself now, but it'll only be effective until level 19.) And your Inspire Greatness' bonus HD (which now hits three targets per use) is also doubled courtesy of the Words, all at the low price of a tiny bit of nonlethal damage on a d10 HD class with healing powers. Interestingly, if you look at Optimization By The Numbers' HP curves, the conclusion is that you need a linear progression of damage with a huge boost around CR 14. Look at what level we get Words.

Also, it's easy to forget this (since no one takes Words for any other reason), but the Words of Creation have more uses than simply boosting Bardic Music. You can use them to Extend any Conjuration (Creation) effect (and you have access to wizard spells, so you've got plenty - interestingly, these are also the same spells Shadow Conjuration can copy (although Shadow Conjuration itself can't get Extended this way), and illumians have a connection to the plane of shadow), boost the caster level of [Good] spells (you have several of these as a paladin, plus many wizard spells can become [good], such as when summoning Good creatures), and perform a number of commanding/binding effects based on the creature's true name (though doing so requires Knowledge skills and two specific spells - this is one area in which you fall behind a Words of Creation bard, but you can still use it against the archetypal lich via Knowledge (Religion). You can have an ally provide one of the spells, but for the one you must cast yourself, courtesy of Sword of the Arcane Order, you can use a scroll - you have enough Intelligence and a high enough CL to pass the check on a natural 1.). 

For a high-level illumian paladin, that's a good complement of abilities (adding some much-needed options to the standard paladin), and a surprisingly good match thematically.

The divine spirit is no Immortal Fortitude, but it's still a damn fine ability (DR 10/-, Diehard, and speeding up your Lay on Hands) which makes you appropriately tough for a hero at this level. Amusingly, you'll probably finish the battle with more HP remaining than your Immortal Fortitude buddies, since they won't be using Immortal Fortitude at all unless their HP is dangerously low. You just block the damage (possibly absorbing some of what gets through via temporary HP effects like False Life) and heal it up. (And you still have your healing spirit to patch the crusader up too.)

Although it doesn't look it, this level has two surprisingly good points falling out of the progression.

First, you’re now hitting your entire team with a single use of Inspire Greatness. Thanks to Song of the Heart and Words of Creation, it’s now providing 6d10 HD, +6 attack, and +4 Fortitude (the Fort save is the one area that Song of the Heart doesn't boost but Words does), in a bonus form that stacks with Inspire Courage AND your Spirit of Combat. You can also use both of your major Inspire abilities in every encounter you're assumed to have in a single day, which is a very nice improvement in stamina.

Second, Practiced Spellcaster has now raised your CL all the way to 17 (18/2=9 +2 Krau +2 Mystic Fire Knight +4 Practiced Spellcaster), and you can cast wizard spells. If, say, you pick up a 9^th level wizard spell on a scroll, you need a caster level check to trigger it, but the DC is only 18, which you'll get on a natural 1. A paladin casting 9^th level wizard spells with ease? Inconceivable! 

Sure, the CL and DC will be minimized (CL 17 and DC 23, like all 9th-level wizard scrolls), but this lets you double-up on (or spam) effects that just need to be cast, as opposed to need to be cast effectively. (Think UMD rogue here but with a far easier time on the checks.) And it doesn't need to be limited to 9ths - you now have access to the entire library of wizard spells from scrolls without any checks involved. (These include all the spells needed for Words of Creation's True Name abilities if you so wish.)

Of course, to do this you need enough Intelligence to actually cast the spell, so without help, you've only got 1st-5th level spells open. To save money getting the 19 Intelligence needed to get the 9ths (since another ability score booster cuts pretty heavily into your arms and armor budget, particularly since you aren't likely to do this every battle), you can simply prepare Fox's Cunning in one of your second-level slots, cast that from a cheap scroll beforehand (using the same idea but a lower-level spell), or chug a (less efficiently priced) potion.

However, it’s not entirely clear if this works, due to the way that spell lists and Sword of the Arcane Order operate. The feat doesn’t say the wizard spells appear on your list, but if they don’t appear on your list you can’t cast them in the first place. Check with your DM first.