...will he/she be treated with hate by the Nords sided with the Stormcloaks? The Thalmors will be kind instead of attacking? Can I walk around freely in Thalmor Embassy during diplomatic imunity and where the Gray Mane dude is kept prisioner? Thinking about to do a Pure Evil Thalmor Inquisitor Nord Killer.

1. No

2. No

3. Yes/Never tried, but likely No.

Hevnoraak may notice His name being used by Necromancy and be able to provide an answer to 4 that is definite. Cider has done a similiar Roleplay and also may provide a Butter Answer

During Diplomatic Immunityhttp://www.uesp.net/wiki/Skyrim:Diplomatic_Immunity#Hooded_Thalmor_Robes_Method.

All others are not possible unless you have mods (of which I know of only one that might allow you to do so in those other instances)

Well, I found some Stormcloaks traveling and they attacked me. Didnt find any Thalmor Yet.

I think the Thalmor on the road are always scripted to be hostile when you talk to them. Everyone else will, oddly, not really notice so you're free to walk around Windhelm dressed in the clothes of their hated enemy with only a few nasty comments shot your way. As for the Embassy? Well, you can wear the disguise to a certain point. Once you get past that point, every other Thalmor there want you dead.

Thanks, Bethesda should've guessed that we would try to play as a Thalmor with an Altmer.

It was clearly intended for the character to be able to disguise themselves in the embassy. To a degree anyway. If you play as Argonian or Khajiit and try sneaking around in Thalmor robes, the guards will make comments like "Is that a tail?" I just wish they had expanded that out into the rest of the game world, along with the other faction armors.

The Thieves Guild armor, the guards do some comments about it.

Yeah it's really a shame you can't properly pass for a Thalmor.

Doubly sad that Thalmor wasn't a joinable faction for those of elven races.

If you are on PC you can add player to the thalmor faction and they won't attack you and revert to standard faction npc dialogue. I did this with a character and it is a nice little change up in the game. You can pretend you from another faction within the Thalmor that is infiltrating Skyrim for their own reason, or that one of Elenwen's superiors sent you because they were unhappy with her progress in Skyrim or they also mistrusted her. I envision a society like the Thalmor's being rife with infighting. It is also the same thing the Nazi's would do, send secret SS agents disguised as regular officers to keep an eye on the other branches of the military. When at the Thalmor embassy on the main quest, I sort of sneaked around to make it feel realistic. Once you add yourself to the faction though there is no way back I know of, but regular people don't treat you any differently.