First of all, you are labeling characters randomly off as low tier. While characters like Kirby and Mac are more widely agreed to be in the lower end of the tier list, putting characters like MK and DH as low tier is more controversial.
Yeah. The Levin Sword change is kinda uncalled for. While annoying, I wouldn't really call Robin a character that desperately needs buffs, especially giving his rewarding moves and high learning curve.
The main issue with Mac is that his grounded jump doesn't reach the lower platform of stages. This causes him to be bodied by stage counterpick. A simple fix to that is already a huge boon to him. Giving back a few of his grounded KO confirms that he lost doesn't hurt either.
I wouldn't say he needs a significant damage boost, but his reward does need to be tuned up a little to compensate for his mediocre air speed and stubby feet.
First of all, MK's special moves putting him into freefall has been a thing since Brawl, so no, that doesn't need a change. Second, what makes you say MK is low tier?
What gives you the impression that their hitboxes are "ridiculously small"? The duck's attacks gives them about as much range as TLink and YLink's sword attacks. Either way, what makes you think that simple fact ruins the character, especially since Pichu exists.
Lack of armor doesn't cripple the character at all, but it indeed creates some annoying matchups, especially against YLink. I do agree with more armor on the side B.
No argues here. Two big bags of huge potential (whether it is from Attack Canceling or Desync Combos), but is hindered one way or another. Luna tethered needs more damage, while Nana should have better AI.