Can a Paladin (or any class without Sneak or Hide/Invis) still...
 

...kill guards in High Keep, tank their reputation, and still be able to sell/bank there?

Two NPC's I'm looking at in particular, Bank Clerk Jaylin and A Clerk, are supposedly on Qeynos Citizens faction and not High Keep faction, and killing High Keep guards doesn't hurt this specific faction, so even if I become KoS to High Keep guards and citizens, these two should still do business with me, right?

High Keep Guards are corrupt, and I plan to wage war against them, the Freeport Militia, and the Corrupt Qeynos Guards (fully supporting the Knights of Truth) for roleplaying purposes. But I need to make sure this is actually a viable solo leveling/money spot as a Pally before I start killing my reputation there (and make sure it won't hurt my ability to do SoulFire later).

Yep, Tarn Visilin, I believe, in the little library room is indiff to all.

Every class can invis, it’s just some do it through click/potion.

Also note you can still use a bank while kos. Either camera round the corner or just take a beating during your bank meeting.

The highkeep vendors will never become kos. They will bottom out at dubious I believe. But I am most definitely kos to the banker after killing guards for several levels. If you wedge into the corner (where Bosec stops in this path route) you can bank without aggro. Or lull the banker and bank at max melee range.

As someone said above. Tarn Visilin will always trade with you.

I guess I'd better not mess with High Keep faction then, as I need to stay indifferent to the Banker for a quest turn-in, in relation to SoulFire.

I hate giving up the High Keep guards because they're such good, consistent money. It's hard to match the money you can make off of them at 25-35. I don't know where else I could go for those levels that would generate the same plat (Sisters in OoT are always camped).

However, that does raise another question. I was planning on going straight to Paineel after hitting 18-19 in Lfay farming Nybright Sisters, killing the guards there. But I've already tanked my Heretics faction, it can't get any worse.

So how do I get a key and get inside there now for fighting guards?

Just kill in the Warrens for awhile (but don't turn in the Kobold Molars in Erudin).

How many kills will it take? Do the Kobolds really only give 1 point per kill?

Excellent question that I do not know the answer to ... except that it will definitely take "a lot" :)

Beyond that, hopefully someone more knowledgeable can chime in.

Well, when/if I get my key, it better not be something temporary or that gets erased at some point once I start killing Paineel Guards.