You say you've been convicted, but I assume you mean charged/arrested. If you recently tried to tender a plea, your attorney may be able to maneuver the case to a different judge. Perhaps he or she will offer you a probationary term. If the theft was to support a drug habit, you could show good faith by entering treatment. Good luck.
Yes, there is no time limit for a murder prosecution. But you say there was no sufficient evidence to "convict him" before. Was there a trial back then? If so, double jeopardy would bar his prosecution.
Yes. Second offense for a Class B poss w/intent charge has a 2 or 3.5 year mandatory minimum depending on the substance (crack, cocaine and meth are 3.5). It is likely 3.5 for your boyfriend if he is being offered 2.5. Essentially, the DA is agreeing to reduce the charge to a first offense in exchange for a plea.
You've asked a lot of questions. The only one that can be answered based on your post is on the statute of limitations. There is no time limit on when one can bring a motion to vacate a plea (which is called a motion for new trial). You can bring such a motion 50 years after conviction if you want to, though it obviously gets more difficult to make the case as time goes by.
The previous answers are not entirely accurate. If a person is charged with a violation of probation, the judge can order him held without bail or released on personal recognizance. However, in Boston, a judge also has the option to set a cash bail in any amount, in lieu of detaining the defendant.