I think starting with small fire Mario is a good idea : the setup is not hard, your hitbox is the same as in Any% and you don't need to duck on tight parts. Really enjoyable.

if you landed on the axe frame 147 yes. If not, for example 149 it will be -2 (8-3)

It's the value under the "x" left from "FRAME". You can calculate your 8-4 score by counting frame by frame or looking at the patterns. If you land on the axe frame 147 without fast acceleration in the last room, then the value will be your score.

Oh well it's SMB Special, there's no 8-4 IL category lol

All levels exept 8-4 are limited by the framerule system of the game so it's not really useful that each level has his own IL category. You can be the faster in a level but there will always 0.35s gaps. 8-4 IL category exists (https://www.speedrun.com/smbce#8-4_IL) because the time is counted frame by frame on the axe.