And to be honest, I find that hugely restricting and overly reliant on the status quo of what makes a Mario platformer a Mario platformer. Mario platformers aren't as heavily reliant as blocks as you're making out to be: 3D games such as Odyssey or even 3D World have a huge less focus on grid-based block gameplay than the 2D platformers, and I fail to understand why the 2D platformers have to exactly maintain that. It's not like the foundation of the 2D Mario platformers rests on the ability to place tiles and blocks. Let me grab a random screenshot from New Super Mario Bros. games to prove my point:
See? No blocks here. You can easily make an organic level design without relying on the grid structure as using assets here, and character size wouldn't matter as much. In fact, lack of an organic feel to 3D World and NSMB games in their level design when they rely on this grid-based system is one of my biggest complaints about how vanilla these games are. I personally hate the artstyle of using sprite-based, tile structure that the New Super Mario Bros. games rely upon, because it lends itself to a rather outdated and hideous design, especially when they also rely on 3D models for characters and backgrounds and they mesh together horribly. Not to mention, characters in NSMB games already go through clipping issues when they duck through corners and attempt to stand up in the middle of that.
Also, let me introduce you to this minigame, Coin Cache, from Mario Party 5, which involves characters hitting blocks in a similar fashion to how they do in the 2D platformers.
Showing you that, yes, a playable Donkey Kong in a 2D Mario platformer is entirely possible, just like making a good 2D Mario platformer is possible or a 2D Mario platformer that innovates: it's effort, Nintendo is shown to pour effort where necessary, showcasing this off in Odyssey, and designing a game that accommodates more playable characters is nothing Nintendo can't do at all.
EDIT: I also forgot about Smash Bros, which also dealt with your issue of having Donkey Kong hitting blocks Mario-style. And there wasn't any issues with that, specifically concerning where the whole "center mass" of the character is.
Sometimes, the easiest solution of maintaining the status quo because it has worked is not the best solution. When I think adding more playable characters is worth it, I think it's worth tweaking some of the stuff around to allow for it. That's how games evolve. That's how games get better.
I really think you're overcomplicating a playable, quadruped Donkey Kong in a Mario game.