Your post wasn't entirely clear because you went off on five tangents before returning to the b-air shieldpoke thing. Nothing I said was wrong, it just wasn't relevant because I misinterpreted what you meant.
The scenario you came up with was pretty unrealistic. Even if you n-air, he could just wavedash outside of shine range and grab you. B-air is also more likely to catch someone trying to escape behind you (we don't live in a frame perfect world, so depending on when Marth starts his wd and you start your b-air, you could easily hit him out of it).
Anyways, in that situation wouldn't you either go further into his shield and shine grab, or do an early/mid aerial in case he rolls? What's the point of doing a late n-air in place on the back of his shield?
I NEVER mentioned early bair lol
The scenario isn't unrealistic either. Shine is invincible at the frame it hits, so it being out of range or not doesn't matter (since he reaches out his hand). You could also spotdodge.
And you don't seem to understand how this works. Early bair (that would possible catch marths wd) will get shield grabbed by marth. Late bair, is vulnerable to wd oos after shine. If you do the bair early enough so that it catches him, then he could have stayed in shield and grabbed you. Its a mixup, and your reward for hitting one bair is nothing compared to his touch of death grab.
Bair would be more likely to catch someone wding away, but wd away is fast enough and travels far enough that its not even close to mattering. Nair reaches where it matters, WD oos past you.
What I would do depends on what I think he will. If I think he wants to wd oos past me, I'll mixup between dash away, raw grab, doubleshine, shine grab and waveshine on his shield. Early aerials aren't good against marth, since you can just jump over his grab anyway. Late nair gives you a chance to punish grab (after shine), any jump oos, most WD oos', spotdodge, reduces their shield and it puts you at FRAME ADVANTAGE if done late.
And I already stated, you shouldn't be shield pressuring him with aerials. Nair is just by far the better option if you want to.