Age-old dilemma: Reneging MM Offer for BB?

I understand that there is a thread of this kind every year, but given that 2019 SA recruiting timelines at all firms are insanely early this year, I found myself in a head-scratching situation in mid-September.

My background: junior at non-target. High GPA and good qualifications with relevant regional boutique sophomore internship. Networked hard at all BB banks throughout the entire year.

Came late August: I had a superday with a reputable (definitely not household name in the US) MM bank and was fortunate enough to land an IBD SA offer for their NYC office. I love the people I met there (almost no alums from my school, unfortunately) and am grateful for their offer, but can't help but feel missed out on upcoming BB superdays (e.g. Citi/BAML/BarCap) and concerned about my exit ops (my dream is to exit to MF PE and/or attend a top MBA program).

My questions:

• How frowned upon is it to continue recruiting with BBs after signed offer with MM, and possibly reneging upon landing a BB offer?

• If I go through with MM SA program, how do I ensure success in BB FT recruiting, which I heard is almost impossible if you didn't intern at a BB?

What would you do?

I'd appreciate any constructive insight!