I don't like brains. There's just too much going on, the system is irreducibly complex. Neuro-anything is a branch of science that will continue along like a thorn in our sides, never progressing as fast as we like. I think your coordinators/advisors are just dissuading you from having a go at it because you'd have to take more than a couple extra biochemistry classes, but you already have a track record that says "hey, if I really wanted to throw myself at this problem, I could clearly have a huge positive impact".

Of course you "have a chance" at getting into either of those institutions. How badly are you suffering from imposter syndrome, bro? Seriously. I think you're very well suited for IGPP, a very broad scope of research, and it's undeniable that you have a knack for analytic theory. If you're interested in doing more experimental work, you should have ample opportunity for it after your formal schooling ends. IA also sounds like a great fit, but it sounds like you're not going to make as many contacts outside of astrophysics in the event that you find yourself unhappy in the field (insert field theory jokes here). Consider also that sometimes, you just don't "click" with a certain group of people, so I would encourage you to visit these places and meet with the scientists you'd potentially be working with. Visit as many times as you can. But yeah, that's a really tough choice, sorry, I can't offer too much more insight into IGPP or IA without doing oodles of legwork.

Devac, ask yourself, what makes you happy? Because that's the thing you need to throw yourself at. Not what your advisor or whoever thinks is the most prestigious or difficult. If you can figure out exactly what makes you tick, and then get paid to do it, your bed is made. You might be lucky, and a wide variety of topics may suit your fancy. Cheers to that.