At level 40. Will pacify work on dawnguard in order for me to buy/sell stuff in the keep as i sided with vampires.

Do these spellls work at higher levels?

It should. Good thing you're using Pacify and not Calm.

Are you talking your overal level of 40 or your Illusion level of 40? If its your Illusion level and you have the Kindred Mage perk, then it will work on certain members of the Dawnguard but not higher ups like Isran.

Character level 40. Seems like it stops working/similiar to raise dead spells as enemies level up with you. Anyone tried it at level 40 or higher?

If you are mastered Illusion mage and have all the right perks. The best calm spell, when duel wielded, will work on any NPC that isn't immune to magic or Illusion magic.

how about level 70+? will any of those perks work at that stage?

Like I said. For all. What I've mentioned will literally go up to lvl 86 in the end. But requires lvl 100 Illusion and a lot of perks. (Note: Enemies/NPCs don't go beyond level 81 last I check).

Though then again. 86 was for the Expert rage spells, if I'm correct. I'm only assuming the Expert calm spells also goes around there. (Note: Use the Expert level ones as they can be duel casted. The Master ones do not so they can't really go beyond 50, I think. Also note the 'max level' is hidden within game but it is there, trust me).

~Edit~

I've decided to check out the max for the three expert illusion spells.

Frenzy - 79 Max when duel wielded and with all required perks. (Not 86)

Pacify (Calm) - 83 Max when duel wielded and with all required perks.

Rout (Fear) - 88 Max when duel wielded and with all required perks.

There ya go.