Do power attack perks in Skyrim apply to sneak power attacks

While wielding a daedric dagger, having 100 one handed(all perks) and 100 sneak(all perks), the daedric dagger's base damage showed as 33. From the dagger's sneak attack(only 15x damage) it caused around 500 damage. Then I applied a power attack and it caused 1,000 damage. Then my orc used berzerk and a single strike caused 2,000 damage.

I could do some more testing (like getting shroud gloves, dual wielding with a power attack doesnt do a single hit unfortunately), and having bonus one handed damage enchants(it modifies the displayed base damage btw), but I think this is enough to show that multipliers stack (15x + 2x = 30x + 2x = 60x) instead of combine(15x + 2x = 16x + 2x = 17x).

I was wondering if by attacking bare handed, you increase your 1-hand (or 2-hands) weapon skill mastery.

I played through the start of the game unarmed (Khajit), and never increased my one-handed or two-handed skills.

[...]do the Khajiits's +15 claw bonus mean that every one-hand weapon below damage 15 is useless?

If only comparing pure damage, yes.

Does, as example "Backstab" skill bonus apply just to any 1-hand weapons, or to barehand too?

The sneak perk Backstab applies to one-handed weapons, which bare-handed is not.

Can I, say, do a stealthy istant kill from behind when unarmed? Like breaking the neck maybe?

You can sneak attack while unarmed (and with a two-handed weapon) for extra damage. If you do enough damage to kill the opponent in one shot, you may be treated to a special animation. IIRC, there's an unarmed neck snap one.