Is this possible? Thanks to an upgrade I don't have to be afraid anymore that I can not play the game when I don't have access to the internet (play in offline mode) but even with that my computer need a Lot of time before even coming up with this option.

So, if I say this right: can I play Skyrim 'behind' Steam?

No.

Anything else is not legal. You need to have Steam running to play the game.