Fox and Falco's Blaster don't interact with any hitboxes because they're transcendent. So, it'll go through anything from a Mario jab, a Link Ftilt, the disjoints on Captain Falcon's Up and Side Smash, and any projectile like Luigi's Fireball, Mewtwo's Shadow Ball, and Ryu's Hadouken. This is both a good and bad thing. Good thing is they can bypass projectiles, but the bad thing is if they shoot too much, too close, or in Falco's case, shoots at all, and it bypasses a charged projectile.
For Corrin's Side Smash... During the charge, I believe the charging hitboxes are recoilless. Some people say trample, some people say... er... Anyway, the hitboxes will continue despite clanking meaning if spaced right, someone could clank with Corrin during her Side Smash, suffer from recovery, and she hits them with a charged Side Smash. I have a replay with that happening against Falco's Fair, but it's on the 3DS... Someone could just try to replicate the situation, though. Also, I think at a certain point, she can get hit since I landed with Falco's Fair which calls for Falco's landing hit and she got hit out of Side Smash.