KKMan

You are probably on their informal waitlist. You might receive an offer even on 13-14 of April, contingent upon their first choices declining the spot. Conversely, you might receive a rejection as well, if they accept it. If I were you, I'd wait at least until then. I really wouldn't recommend that you do that. Ethically speaking, you would be ruining that spot for someone else, someone who might be considering that school as their dream school. But more importantly for yourself, you should know that postgraduate research community is pretty small, and everyone knows each other in your prospective field of research. You will be in a community that constantly interacts with each other in conferences. People talk, and when your name comes up, people who's offer you've accepted, then declined will definitely talk about this. That will affect how people see you within the community and it may also affect your future prospects if you are known as the person who pulls this kind of stuff. If you want to be proactive about this, you can contact the schools that have been silent and tell them that you have another offer that you have to respond to before April 15th and that you don't want to lose that opportunity, but you consider their school as your first choice and you will accept it immediately if an offer is made. I know waiting is exhausting but you've made it this far. You wouldn't want to do anything that will harm you in the future based on an emotional decision. I hope this helps.

I think it's really nice of you to consider declining your existing offers, given that you have an admission you are set for. If you have 4-5 or more offers you could consider narrowing it down to 2-3 of your top choices and wait until you get a confirmation from your top choice that you are comfortable with. As someone who was waitlisted myself, I can say that as much as it would help other people a lot if you decline your other offers, you worked this hard to get yourself to where you are and its well within your rights to hold on to your other offers until the end (April 14-15) if that makes you more comfortable.

I really don't think so, that's really unheard of. Even though they might be legally allowed to do that, at the postgraduate research level people are aware that the communities are very small, everyone knows each other and the ethical backlash of this would be huge. I think they would be more likely to not make further offers and/or reduce the cohort size for the upcoming year, than to take back the offers they already sent out. Having said that, I don't want to cause you concern but if you haven't received an official confirmation of funding, but only received an admission, that might be something you might have to worry about. Then again, I think they would make their funding arrangements for further offers, not existing ones. I do hope it all works out well for you.