Diversity Events After Accepting Offer?

I'd say your decision on this should depend on a few things. 

1. Why exactly are you afraid of not getting a return offer? At some firms (BBs especially), you are virtually guaranteed a return offer unless you are consistently not cutting it, or are just a really bad fit. In the first case, IB may not be a career path that is well suited to you anyways, and in the second, it would probably be hugely beneficial for you to lateral to a firm where you would fit in better. If you are heading to a place that has historically had a very high offer rate, I would relax a bit. It is very natural to worry about future unknowns, but if this worry is without basis, try to let it go. On the other hand, that doesn't mean you shouldn't be realistic if you are headed to a firm or group where returns are genuinely a toss-up. If your worry stems from some known, significant chance that you won't be asked back, quietly and respectfully attending these events to give yourself a safety net of connections at other banks might be a good idea. 

2. Are you happy staying at the firm where you have your current offer? If the answer is yes, I would focus your energy on putting your best foot forward this upcoming summer. In the event that you aren't asked back, you can deal with what to do later- plus, you'll have peace of mind knowing that you haven't done anything to promote the idea that you don't want to be at your current shop. If you're considering lateraling or think you'd ultimately like to end up somewhere else, the risk of attending these events is worth it. 

3. As best you can tell, how much would your firm care? I'm a diversity hire, and prior to the start of my IB internship, I selectively attended a few summits/events. Most were for buy-side roles, but one was for a bank similar to mine. Almost everyone else attending these events had an offer at some different firm, and in general, people with good offers who just wanted to keep their doors open were very careful and specific with how they presented themselves (here to connect and learn from others in the industry, NOT here because I'm interested in recruiting for X firm). At the same time, though, my firm isn't really the type to ding someone for attending a different firm's networking event. Other interns and I talked pretty openly about our backup plans, and I'd never heard of someone losing an offer for quietly flirting with competitors. That being said, summers also stuck to a strict narrative- over all else, our firm was where we wanted to be, even if that was blatantly false. 

One more comment to calm your fears: Just want to throw in here that I think it's really rare that an offer decision at the end of the summer is a total surprise. Most people I know who haven't been asked back understand that they aren't getting a return offer by the last few weeks of the program- either because someone has blatantly told them, their reviews haven't been positive, or because they can just feel it. If you aren't being asked back, it is incredibly likely that you'll have a few weeks of time during the summer to start to put out feelers and network before throwing your hat in the FT-recruiting pool. You shouldn't feel like you need to extensively prepare for the worst this far in advance.