So I am coming back to this game after some break and will try to properly finish it if I will have enough free time. I have the following questions:
1, What are the best accessories to have?
2. Are there AC accessories, and if they are, does their AC bonus stack?

I am open to spoilers.

It really depends on the character's role.

A Cleric or Bishop is going to want something to boost her Holy attack, as that affects the power of healing spells. Note that multiple bonuses do not stack. It may be worth having two such accessories in her inventory, one with a bigger boost (like the Rainbow Charm if you manage to get one (Ibag Tower 16F-20F rare drop) or the Syhpon Ring (steal from a certain enemy that appears in Ibag Tower and is quite dangerous)) and one that doesn't make you weak against enemy spells.

A Mage or Bishop will want charms to boost the power of attack spells. Remember that you can change equipment during battle without using up your turn. (Or, you could Spirit Pact a Don Mags, who has high enough bonuses to not need equipment boosts, but you may need to reload (after Contract but before Spirit Pact) until you get an EX Skill you like (like Magic Essence for example).)

I believe AC accessories do exist, and that they stack, but in the late postgame (Ibag Tower) AC isn't that useful unless you can get it down *really* low. Unless the character is a Brawler or Ninja, this will mean using alchemy to improve the AC of every piece of equipment.

If you can find something with good Beheading resistance, that can be useful. Getting 100% Dark resist will prevent you from instant death magic, but not from beheading physicals or death breath.

Turn recovery from equipment/alchemy is too weak to be useful post game. (When starting out, of course, you will find it to be incredibly valuable, but it doesn't scale well with enemy difficulty and your HP.)

One more (unrelated) reminder: Status effects and instant death attacks are useful in this game, though you will want to check the bestiary to see what enemies are weak against.

Getting 100% Dark resist - is it possible? And how?

Also I tested and it seems that Turn Recovery accessories don't stack, resistance accessories don't seem to stack either, but AC accessories do stack.

Yes. +3 Dark Seal provides 50% Dark resistance; temper with a material with at least 25 AP that can boost elemental resists and you can increase it to 100%. There may be other suitable equipment, and some monsters (those with 100%+ Dark resist) will have that much resist after being Spirt Pact-ed into your party.

Aha. So I guess Dark Seal will also be my accessory of choice. Everything that helps against Instant Death is a good think (I never really liked that mechanic in any RPG). Btw, do resistance effects from alchemy tempering stack?

Resistances from tempering do not stack.

The Dark Seal makes you weak against Holy attacks, so you won't necessarily want it all the time. Also, there may be other accessories with Dark resistance. Remember that you need 100% to block instant death spells, and it won't help you against other instant death attacks.

Beheading resistance helps against all forms of instant death, but it is harder to get to 100%; on the other hand, values less than 100% make it less risky.

Remember that if you have a level 26+ Alchemist, you can undo alchemy and get the material back; if not, you can sell and buy back the item to remove the alchemy effect, but you will lose the material.

Not that many Holy attacks though. And usually not very deadly. Well except the Afun Zar Light Cave bottom.
And sometimes Immolarati from a cleric-class mob I guess, but that's mostly a post game thing.

I'd worry about the Instant Death spells more, they are far more common.

Actually, I believe Immolarati doesn't actually use Holy attack/resistance. Also, some enemies have a similar attack that is enemy-only (and not usable by monsters summoned by the player).

You might be right about Immolarati, as it probably deals damage proportional to health.

And yes, the monster only skill is Strange Miasma Wave. Deals 1/2 current health damage to all party. Quite devastating and unfortunately much more common then Immolarati. Some monsters love to spam it several times per turn. Good thing the AI doesn't seem programmed to exploit low HP and follow up with a damaging attack that will actually kill - still happens, of course, but not as often as I would expect.