A theory of fluff: Aereni diplomat with ties to the Valaes Tairn
OK, so here are some thoughts on this:
Elves in Eberron come in roughly three flavors: original Aereni, fancy Khorvaire, and spicy Valenar.
The Valaes Tairn elves of Valenar are extremely well-known as mounted marauders.
Valaes Tairn come from Aerenal; all Valenar warhorses are bred on Aerenal, all Valaes Tairn are born and raised on Aerenal, and so on.
The Valaes Tairn are following a tradition of war dating back to ancient Aereni expeditions to Khorvaire.
You are an Aereni elf, but the difference between the Aereni and the Valaes Tairn is really just a question of which ancestors your venerate more (the Undying Court vs. the war-heroes of Xen’drik). All elves on Aerenal venerate the Undying Court, and all elves everywhere are descended from the heroes that the Valaes Tairn venerate. There is a lot of overlap here.
I am now imagining an Aereni elf who approached the Keepers of the Past, interested in his or her ancestral patron—the Valaes Tairn believe all elves have one—and was surprised to learn that his or her patron was not one of the heroes who died in the fighting in Xen’drik, but one of those who protected the exodus from Xen’drik to Aerenal. Maybe that ancestor actually made it to Aerenal, and is actually still a member of the Undying Court. This led the would-be Valaes Tairn to serve the Undying Court despite his or her warlike inclinations. The books do not make any official mention of such ancestral patrons, but then PCs are often a little unique.
Mounted combat and a divine fighting technqiue
Anyway, my point here is that horse-elf-ship is an Aereni tradition; the Valaes Tairn are the most serious about that tradition, but all Aereni are descended from that tradition. Aerenal is also apparently a pretty good place for raising horses, and there are horses there. You can definitely make Aereni elves pledged to the Undying Court who nonetheless exhibit some of the warrior-skills of the Valaes Tairn.
I bring all this up because being mounted is decent direction for a warpriest to take. The greatsword battler divine fighting technique allows you to combine charges with the Vital Strike line. The Vital Strike line generally sucks, but combining it with mounted charges goes a long way to improving things.
In Golarion, the greatsword battler divine fighting technique is associated with Gorum, a god of battle. Again, the Tairnadal religion is probably the closest thing on Eberron to that—but Aereni elves and Valaes Tairn elves are not very different. Associating greatsword battler with Tairnadal is entirely appropriate, and an Aereni elf could follow the Tairnadal path even if not born into it directly. And if the katana is your ancestral weapon and “part of your soul,” Tairnadal would have you use that, not a greatsword (a greatsword is a better weapon anyway, so there is no balance concern here). The feat requires matching the alignment of the deity—Gorum’s is Chaotic Neutral—but the Tairnadal religion is a form of ancestor worship, so an Aereni elf of any alignment could easily make sense. Officially, the “spirits of the past” as a collective “deity” are true neutral.
With this, you want a decent mount—you could use the divine commander archetype to get one that scales like a druid’s animal companion, but the rest of the archetype is worse-than-useless to you, since it replaces crucial bonus feats, particularly the 6th-level bonus feat you need to take Vital Strike “on time.” If your GM is willing to give you some wiggle room on that (maybe move the blessed mount feature to 9th, replacing the 9th-level bonus feat?), I would consider it, but barring that the Blood Steed trait is a strong consideration for you.
Diplomacy without Charisma: learn some humility
Since you are also a diplomat, and you have no need for Charisma, Creed of Humility is important for using Wisdom for Diplomacy. This does not replace your Charisma modifier, unfortunately, so having particularly-poor Charisma will hurt you somewhat, but it’s still better than having to pump your Charisma which does literally nothing for you. The Creed of Humility could be quite appropriate too—if this elf’s patron ancestor was not a great martyr-hero, but rather a quieter, perhaps more practical or disciplined warrior, who forsook suicidal glory in favor of just making sure the exodus got through.
On the other hand, multiclassing offers a superior alternative to Creed of Humility: a level of inquisitor with the Conversion Inquisition could get you charm of wisdom which replaces Charisma with Wisdom on each of Bluff, Diplomacy, and Intimidate. That hurts you a lot in terms of lost spellcasting, though. Plus it’s harder to justify the Conversion Inquisition—the Aereni are not much interested in converting anyone to their faith (since it wouldn’t make any kind of sense for non-elves and the non-Aereni elves of Khorvaire are not well-regarded by the Aereni).
Feats: a conclusion
With the above in place, you want to get Spirited Charge as well as the entire Vital Strike line. Power Attack also makes sense.
Something like this:
||Creed of Humility
||Mounted Combat, Ride-by Attack
||Divine Fighting Technique (greatsword battler)
If you end up going beyond 9th, Wheeling Charge and Indomitable Mount are desirable for you, and of course you want to get Improved Vital Strike and Greater Vital Strike at 12th and 18th, respectively.
Blessings: ideally, Destruction and Liberation
Finally, on blessings. The Undying Court offers the Deathless, Fate, Good, Protection, and Planning domains, while the “spirits of the past” offer the Destruction, Protection, War, Elf, and “Revered Ancestor” domains (the last is a domain unique to this faith that emphasizes a lot of Valaes Tairn things, including their signature double scimitar). There are no real blessing analogues to the Deathless, Elf, Fate, Planning, or Revered Ancestor domains. Of those with direct analogues, Destruction is easily the best, with Protection or War as the best choice for your second blessing.
If you are allowed to make arguments for other blessings, though, there are some you should consider. The Community blessing is interesting in that you could use it on your mount, who could then use its standard action to aid your attacks: a +4 attack bonus is not bad at all. Strength is pretty decent too, though you might want to consider going for a more overrun-based direction with that. The Liberation and Travel blessings are also really good, since they allow you to ignore difficult terrain and therefore allow you to charge through it.
If you can get into pure homebrew territory, you might consider trying to argue for some blessing that you could use on a mount—say, a swift-action blessing that upgrades the mount to animal companion status for a minute? I’d allow it, anyway. Certainly sensible as a Revered Ancestor blessing.
Don’t waste time with the Healing blessing; you have healing ability, but you are a warrior first-and-foremost. Healing is mostly going to be zapping a wand of cure light wounds between fights. Stabilize is good for emergencies, maybe a one-off cure light wounds for a truly dire situation. And even if you prepare a lot of cure spells (which you absolutely should not do), the healing blessing offers little real benefit.