This is probably one of the main thing that bug me in-game, whenever i fight something, my companions aways semm to get in my line of fire or get in the way of my swords, wich results in then falling flat down on their asses and die. I wish to know if it's possible to actually be unable to damage then trough your weapons DURING A FIGHT, so when they are in battle stance i can't damage then.

When the modding kit comes out, it will more than likely be extremely easy to give a companion an enchanted item which provides 100% resistance to fire, frost, magic, or lightning. I haven't seen all of the enchantments in Skyrim yet, I don't know if there's a resist physical damage enchantment option (there MIGHT) be. Even then it would be problematic because then they'd be immune to said enemies as well. I've played Oblivion with modded companions before, none of them were immune to my attacks. I assume the only option is to make a mod that marks them as essential. I could be wrong, but I never heard of any mod in the past where companions are immune to the players damage only.

I get particularly annoyed by the fact that I get sneak attack bonuses on my companions. The whole idea behind sneak attack bonuses is that 1) your target doesn't see you, and that 2) you get the opportunity to carefully plan an attack on them where it will do the most damage. Both of those things are false in the case of companions: your companion knows you're there, and when you hit them, it's a case of friendly, inadvertent fire -- the exact opposite of a planned sneak attack.

And yet, again and again I get the "Sneak attack for 3.0x damage!" and see Lydia's corpse slump to the floor. :/

That would just make then incredibly overpowered ._. I just don't want then to get hurt by ME, not from normal enemies, that would just break the game.