ikaro:
I have a question about the FAA issued 250kts below 10000ft restriction. In the FAR 91 it says only the administrator can change this, then, can I accept a clearence to maintain 280kts at 5000ft given by ATC?¿
No you cannot, not under any circumstances. If you could it would state "unless otherwise authorized by ATC."
Th only exceptions are listed in the regulation, such as heavy takeoff requiring greater than 250 once cleaned up. Some operators take that interpretation; some don't.