1. If you have 1 carpenter and 1 day worker at a castle and you move the carpenter or build the castle, so the day worker is alone, does it leave immediately (allowing it to be used again)?
Yes. That's correct.
2. If you can't pay back all your loans at the end of the game, what happens?
The player cannot take the point from the castle tile.(see Loan section)
And the following is lost from my rule. Sorry.
A remained loan marker is -2 points. When you have two loan marker in the board, it's -4 points.
I will update these points as soon.