I just finished the quest to end the rebellion, got the princess and Aire, can talk/enter Williams mansion and got to act II... I thought that after becoming the mayor the loan is cancelled or just optional, but the bank keeps coming to collect the debt or game over if there is not enough money.

 Sure, its not hard to get the money, but it is supposed to be like this?

kinda sure that all the work to become mayor is to get rid of it.

No, it absolutely isn't supposed to be like that.

I've never had that issue myself, so I don't know what happened, but I suggest either starting over again, or maybe getting a fresh install of the game.

Edit: Had been a bit since I played, so I completely forgot but!

If you're playing on Hard, the Loan quest isn't canceled upon Prologue completion. You're most likely playing on Hard rather than Normal, which is causing your issue.