The MU isn't good but let's not exaggerate.
I disagree that her projectile is irrelevant in this MU. I think you're underestimating how good the move actually is. Even a partially charged one is enough to stun us for long enough for her to get a follow up on it, and that is definitely spammable if the Corrin wants to play that way. Our fully charged CS will clank with her fully charged water ball, and if it isn't fully charged and doesn't clank with the water wall, it can possibly still clank with the bite.
This move is not good or anywhere near spammable. We can't complain about followups off the move when there isn't a good excuse to ever get hit by it. To put it into perspective, Corrin's neutral b is laggier on the ending than Samus' uncharged neutral b and significantly laggier than the ending of ZSS' uncharged neutral b. i.e. uncharged fang shot is not spammable at all. It is also ridiculously reactable (it is an already mediocre frame 15, but only hits on frame 15 if you are literally right next to Corrin since it is a projectile) and ridiculously slow in the air. And the reward for landing it is usually 4% no followups.
Mid charge fang shot is even worse, because obviously the startup is slower. But mid charge is the only way Corrin will get followups sub 110%, and I shouldn't even mention followups when nobody should be getting hit by a frame ~30 (much more than that when range is factored) projectile in the first place.
Samus has way stronger projectiles. If some FG Corrin is spamming fang shot, use missiles (10%, definitely an advantageous "trade" though missiles eat through various levels of fang shot) or charge CS and shield the highly reactable projectile, then keep charging during the ridiculously high endlag on the move. With fully charged CS the Corrin will only use fang shot if they want to take 25%. The bite never does more than 10% until full charge, and fully charged bite comes out on frame 85. So if you see Corrin start fang shot, you can fire CS on reaction and there is zero risk of CS clanking with anything--it is a guaranteed 25% damage and possible KO.
A tourney Corrin will never ever use fang shot against Samus, and will probably never use fang shot against the majority of the cast. Just like Zero Suits rarely use neutral b. Corrin is at a huge disadvantage for using the move, especially against characters with chargables.
If she ever gets in on us we are combo food for days at low percentages.
Samus isn't combo food to Corrin. Corrin has very limited, situational combos against the whole cast, and especially against a floaty like Samus who escapes almost everything with DI'ing up/away and jumping. I think an issue in the MU is how much of Corrin's moveset knocks Samus up in the air. I go to the ledge a lot against Corrin (I do against any character who juggles, because the ledge is way better than the air almost always). It could be more accurate to say that Corrin can put Samus in a disadvantageous situation fairly easily (not for free), but not that Samus is combo food for Corrin.
Zair is the only thing we have that can challenge her in the air and that alone isn't good enough.
I almost agree except that we also have CS. Nair can be alright as well, but preemptively jumping and CSing a Corrin who continually approaches through the air can catch them off guard and get a stock. We should also shield always when Corrin approaches through the air, or take another defensive option like shad, roll, walk back, etc. If you are really feeling it then pivot grab but our pivot grab is so bad on whiff. Shielding tends to be best though since we can challenge her OOS.
Her fsmash actually outranges our grab, and comes out faster for good measure.
I don't see many situations where this would really be a factor. It also doesn't come out faster, not that it matters because it's not a situation that happens.
We can't even up b out of shield to punish her aerials on shield most of the time because they have enough range and are disjointed enough that she's usually not close enough to be hit by it, and even if she was her aerials are basically lagless and she can almost immediately follow up from one.
Corrin's bair is safe on shield. The others we can dtilt. Also, for what it's worth, if Corrin doesn't space well, her aerials definitely have lag/her other moves have startup and up b will most definitely punish her. Otherwise, dtilt.
Her dair beats our up b too for the record, so you can't even try to bait an up b kill as she can just dair us.
Also don't see situations where this would happen. If it is really an issue against a particularly dumb Corrin player, then bait the dair.
There are bigger issues with the MU, some of which you already pointed out, like Corrin's disjoints, juggling, side b, recovery, kill options, and her mobility isn't
that bad. My reply was really to point out the exaggerations and that her projectile isn't good. Not that the MU is easy or 50:50 or anything.