I'm not good enough to execute it in a single segment, but I think the fastest way to get 2-Castle 100% is as follows.

Start as mini, go through the door to outside at 72 Switch to Fire Mario right before the boss fight, finish at 31 (1:09 run). I'll try to get it single segment or at least close to that on video unless someone is able to get it before me.

In that case, the 100% route is completely unrelated to the Any% route because of the 2 wall jumps underground, et cetera (as I suspected). From what I've seen, koffing's statement regarding hitting coin blocks also influences the rng, and since that is the easiest thing to control, (hopefully) today I'm going to pick some (5) events that I control if they happen or not and get the results for what the box gives on 1-2 (if anything).

First box, mushroom+box 2, box before Coin A, triple jumping off the Coin A platform, box before pipe to Coin B. 32 cases, hopefully one of them gives Blue Shell.

Edit: I just started playing around with it and switching the time start to 00:00:01.000 on January 1 2017 gives me blue shell. Guess there's that!

Ok, I'm playing around with TASing it, and I don't think the start time on pushing file 1 or double/triple jumps matter. Granted, I'm new to TASing and I'm running the 100% route, but I'm not getting different results (for example) when I triple jump to get Coin A versus normal jump, or have different delays in terms of picking File 1. @koffingrockz, has this issue been solved since your posts? (Also, is there an NSMB Discord?)

Edit: This is bizarre, I have it TASed to entering 1-2 but occasionally I get blue shell but usually it's 1-up mushroom.

What about for 100%, is there a known setup to manipulate blue shell for that?