Technial v. Fit and who your interviewer is

Fits my experience. Analysts/associates in the 21st century wall street oasis era are total gunners from the best schools who likely prepped a ton to land their positions. Many MDs are older and don't give as much of a crap anymore and care more about your character, values, work ethic, etc. However, I would still prep just in case. One interview out of my many, I was absolutely drilled with tough questions one after another by a very senior MD and I ended up flailing and getting dinged. Always better to be safe than sorry.

^Correct - MD's generally aren't very concerned about your technical skills, since by the time they see something, even if you built the crappiest model, the associate/VP will have had you made enough edits to the point that it looks at least passable. That's not to say you won't get technical or harder questions; I'm just saying that you're far more likely to be grilled by an analyst/associate.

I'd also be a little bit more worried about the S&T MD, since that's a little more of a wild card.

S&T MD is supposedly in charge of the summer analyst program across all the divisions. I'm imagining in that case it will be very fit since his expertise is not in IBD, but who knows.

My experience is pretty similar to other posters - MDs and higher-ups tend to be fit based. You got the interview so they know (or at least assume) you're smart and can do the work / learn the work, so they want to see if they like you. Analysts and associates are a little more hit-or-miss. I didn't happen to get a lot of technicals, but definitely felt like more junior people were grilling me a bit more.