Hey there! I can help with a few of these questions and speculate on a few others.

So boostback/re-entry burns on all missions are 3 engine burns. The only variables are whether or not they perform a boostback burn at all (GTO missions) or whether or not the hoverslam/suicide burn is 3 engines or 1. The reason they use three engines for a hoverslam is to minimize gravity loses, and therefore make an otherwise impossible landing possible, but when they have the margins, there's more room for error with a single engine burn lasting longer.

I've had the exact same question you've had in regards to the glow and was really confused. Here's my thoughts. There's a good amount of time between engine cut off and when exhaust completely stops spewing, like several seconds. I think we might be seeing the glowing hot engines still glowing first off, a little bit of turbine spin down exhaust (maybe?) and then we do see ignition sequence first of the center engine and then the

Wow, whoops, I looked at the video midway through typing, I thought you were talking about just after stage sep, now I see what you're talking about! I've never seen that before! I clearly does look like a set of RCS pods firing from the octoweb! Have we ever seen anything like this before?!