Agreed with most of these. But for #3, I don't really care about the lives themselves--those don't really matter that much. The better approach would be just letting the player do a hard save after every level, instead of every Castle level, similar to how the 3D series lets you save after every stage/star. The 3D series has from 64 let the player save after every star and not lose any progress if they game over. The 2D series has been much more stubborn about this and just stuck to castles for whatever reason.
For players like me, who kinda suck but want to get better at the series, this sucks. Because like, in NSMBU, I can get to like Frosted Glacier or whatever. But then I lose a few lives getting to the castle just due to stupid mistakes/human error and only have like 2 for the castle itself. The castle level is naturally a step up in difficulty from the others, so I lose my lives as I'm trying to learn it and get a handle on things. But then I'm sent back to the beginning of the world, which I don't actually have any problem with and it just wastes my time having to get back and doesn't help me to actually learn or become better at the things I'm actually having difficulty with because those levels aren't the ones I'm dying on and don't necessarily even have the elements responsible for killing me and just entirely breaks up my flow. So, by the time I get back to the castle, my sense of where exactly the problematic part is is off, I'm just more frustrated from having my time wasted, and there's just no benefit to me. And I just have to get back there and manage to beat it on like 5 lives which doesn't really work that well when you're bad at something and trying to get better since that ain't that many tries.
It's also why I liked NSLU much better than NSMBU. NSLU introduced the character of Nabbit who's invincible to most things that aren't pits/lavas/crush damage and is unable to use power-ups, instead turning any collected power-ups into 1-Ups at the end of the level. Relying on him let me beat it easily, unlike NSMBU. However, since that involves relying on an invincible character, it's naturally very unsatisfying. I still enjoyed going through the game and appreciated the level design and stuff, but it's very unsatisfying knowing that why I was successful was contingent on using an invincible character. I want to get better at playing the game and relying on an invincible character isn't conductive to that and while it does let me beat the game it doesn't help me to become a better player at all, which is what I really want to do and there's just no good option with the way things currently are.
So while having stuff like Nabbit is one option, I'd definitely prefer the 2D series to just adopt that element to the 3D series, and just let players have a hard-save after every level they clear. That will actually allow players like me to get better at the stuff that's actually causing us problems and manage to home in on that stuff instead of having to retread levels that don't have anything to do with anything when we game over and just waste our time.
And of course it's not just players like me that will benefit in that case. Since players will actually be able to hone their skills on stuff that's difficult for them, step-by-step, instead of being thrown back, and Nintendo doesn't have to worry about stuff like that, that will allow them to create more difficult levels for everyone as they'll have to worry less about people getting stuck and having to make sure they're able to do a handful of levels and can focus on making each individual level more difficult.
But, yeah. It's just so weird to me that they got it right their first try in the 3D series but they still keep this structure in the 2D series and aren't willing to let go of it. I don't really get what the point of it is other than punishing people like me who already suck at the series and deter us from actually getting better by making it annoying to get back to the things that give us problems, and an easy thing to fix, so it's definitely the thing about 2D Mario games tha annoys me the most at this point.