I'll answer the ones I definitely remember the answer for:
We played Siena first the first time and encountered some problems with the rules. Hope anyone can clear these questions for me
1: The courtesan: what happens if I don’t have enough money to pay? Am I simply reduces to 0 florins? What if there are 2 courtesans and I have only 10 florins? Do I pay for one and the second one stays in the inn?
You pay all your money and remove all of the courtesans. In your example, you would pay the 10 florins you have, then remove both courtesans from the board.
2: The beggar: what happens if I don’t have enough money to pay for him? And I want to pay? Must I take a wooden cube?
I don't remember, but I suspect it works like the courtesans... pay what you have. Only take a cube if you have the florins and don't pay him. Check the other forums for this one.
3: producing goods: if there is already a cube of a resource and I play a card that does not have that resource, do I stay get to sell that available resource anyway?
You can only sell goods that match your card. If your card doesn't have a good on it, you cannot sell any cubes of that good that may be on the board.
4: buying Senesi cards: Can I buy these cards even if my action did not earn me 10 or more florins? If the answer is yes then, does “every time” means that if I get at the beginning of the turn 8 florins and then 5 because I landed on a yellow building means I can buy to cards lose 2 for the first one (8 – 10), and then another 5 for the second one?
I'm almost certain that you can only buy the 1 or 2 cards by taking a reduction in the amount you earn from selling goods. If the amount you earned from selling goods is not enough, you can't buy the cards.
Most importantly, you can ONLY do this as a Merchant!
Enjoy the game!