So i hope that i have been played rigth, but i have to ask this: Can sentinel charecter defend first attack made of 1-player and then 2-player with only one exhaustion?

To defend, you need to exhaust, so you can't defend two attack (unless you played a card effect to ready).

Sentinel is used when a player choose to take the attack undefended but before the attack is resolved (so before revealing shadow and co,). Other players can choose their sentinel as defending character. so the attack is not resolve as undefended.

OH GOD WHY...that means that i have to complete almost all guests what we have been played at the 2-player mode

What about characters that "do not exhaust to defend?"

Then you just need him to be ready to defend, and he can defend any number of time until he get exhaust by a nasty shadow card, bwahaha!