The spacing utility is really valuable, making Corrin's bair one of the best in the game. Marth's bair is really good too though. You could make the case that Marth's is better, I suppose, but I don't think it's obviously superior.
Agreed, which is one of the reasons why I think it's better to compare how characters do in different MUs instead of comparing their moves.
It's easier for Marth to get grabs, sure. It's also easier for Corrin to live longer thanks to being a borderline heavyweight.
Dragon Lunge is arguably Corrin's best tool and easily one of the best side-Bs in the game. I'm not quite sure what point you're trying to make here, though. Sure, remove Dragon Lunge and Corrin will plumet to maybe something like top 30, while Marth likely would be top 20 still without Dancing Blade (maybe). That's hypothetical, but Corrin
does still have Dragon Lunge.
I won't say I'm 100% sure it's even, but I'm 90% sure. It's also worth noting that every notable Corrin and Marth main put it as even.
Last I checked, Marth doesn't actually win +2 against any Fire Emblem character, but just +1 against most of them (not Corrin or Lucina). Guess who else wins +1 against every FE character (aside from Marth and Lucina)? That's right, Corrin.
I would say that Corrin does noticeably (1 step, generally) better in the following (somewhat noteworthy) MUs:
Marth:
She generally does better than Marth vs mid and low tiers as well (Marth generally just wins +1, while Corrin has more +2 wins), with the notable exception of Samus, who Marth is better at dealing with. This is the main reason why I think Corrin is better in practice, because her MU spread is generally better than Marth's.
Top 10 area is a lot more evenly matched, barring Bayonetta, than people tend to give credit for IMO.
Oh, he probably does have more
options, a lot of Corrin's gameplay boils down to using her best moves at the right time. Marth has a better grab game and a better edgeguard game. I think Corrin is a bit better than Marth because she has fewer losing MUs and is a bit better against non-top 10 characters (and a bit better against Rosalina and Sonic in 10, at the cost of being a bit worse vs Cloud and Fox). It's not like the gap between the two is huge though. They're pretty close either way (from around 7 to 15 all seem pretty close to me, anyway).
I think Marth should be somewhere around #12-14, with Corrin a bit higher than Marth. It's hard to say for sure since there are so many moving variables, especially since many of the MUs haven't been explored fully (and probably never will be unless Smash Switch is a port).