I've had a bacterial infection in my throat since monday, and I only have some amoxicillin that expired last march (2012). Is it ok to take it? Thanks

No. You should never use expired medicine. And I am not sure you should be treating yourself. Why dont you give that burden to your doctor? He or she will gladly help. :).
No. Don't take leftover antibiotics. At best, they may not have any effectiveness (some lose effect after a certain amount of time). At worst, you may be increasing your chances of antibiotic resistance. Taking antibiotics when not indicated can cause them not to work well when you really do need them. Discard expired meds (safely!) and see your doctor if you think you need treatment.
No. No. Need to have your local doc reevaluate you.
Yes. Yes, assuming your infection really is bacterial. The more of the following symptoms you have, the more likely the infection is to be bacterial, but none of these symptoms is a perfect predictor by any means.: 1. Absence of cough. 2. Fever. 3. Enlarged, tender lymph nodes under the angle of the jaw. No worries about toxicity with expired amoxicillin. But you need a full course of this med.