Winston
Smash Master
I think shinegrabbing is pretty good when Marth is near the edge as long as he's at mid-high percent at least, since fthrowing him creates a free edgeguard opportunity, and even if he makes it back to the edge he's in the bad position you talked about. Your reasons for not shinegrabbing by the edge are valid too, I just think that it still makes sense to do both.I would recommend shine grabbing vs. every character. The trick is when to do it, and I mostly find myself doing it when I know they have a safe OoS option. For instance, if you are mid-stage vs. Marth, he's probably going to just WD OoS after the shine since you need a very precise timing with nair/dair in order to hit his jumpcrouch. So instead of letting him just reset the situation or try to waveshine and chase him down, I'd rather just shinegrab and work off of that. When I have Marth pinned at the ledge, especially when he's at higher %s, I almost never shine grab because I am much more likely to shield poke or hit him as he tries to cross me up to the center of the stage. The safest option he has is to just WD back onto the ledge, which is just a horrible position for him.
That's my general take on when to shine-grab, but I'm curious what other reasons people have for doing it when they do.
Another thing to note is in your first situation, where you think he's very likely to wd oos away from you, going for the read on that can be an extremely high EV play. If you wd towards him out of the shine really fast and shine again, you'll catch his wd away with a shine.