Congratulations!
To answer your questions:
1. I checked back on my email confirmation from last year that I'd paid the MBS deposit, which says that it is nonrefundable. I think it must say somewhere in your acceptance letter as well, but I can't remember exactly. I don't think anything has changed since last year, but if you want 100% confirmation don't hesitate to call the office - that'd be a very quick question for them! Of course, the deposit doesn't mean you necessarily have to attend Tufts. I wish they didn't give you such a short window to decide, but nearly every program does unfortunately :\
2. I have no concrete way of answering this question as I don't personally know anyone who has applied to medical school while being in the MBS program (though there are definitely people who do!). However, towards the middle of first semester the dean of admissions at the medical school speaks with the class. He told us that he does not recommend MBS students apply to medical school while they are in the program. This is because, for the most part, if you're in this program it's because there are already weaknesses in your application that you're aware of. Medical schools will likely only receive your first semester grades, which may or may not be sufficient. Again, it is possible that MBS students do get accepted to Tufts if they apply while in the program, but you also have to keep in mind that your application will more than likely be compared to MBS students who have completed the program in full.
As a side note, I do know of one student who dropped out of the MBS program a few weeks into it because he began getting interviews at different medical schools, which required him to travel. As I've stated earlier, it is very easy to fall behind in this program. Of course, it's excellent to be getting medical school interviews, but if you cannot keep up with the work and do not do as well as you would otherwise it can hurt your application. I know there must have been students who apply to medical school successfully while in this program, but it is not recommended. If you do decide to apply just absolutely do your best to stay on top of your class work while you interview!