Which direction should I learn the animation system if these are my requirements?

This will probably end up as a big spiel, but I don't know what else to do at this point.

So, some background:

• I've been using Blender for game development for several years, so I'm not new at the concepts.
• I've been self-learning Unity for a few months now.
• I do all my modelling in Blender.

My learning process involves setting a goal and learning whatever's needed to accomplish it. Currently, that goal is "make something that resembles Super Smash Bros." (i.e. a platformer fighting game). I successfully implemented the start of the fighter scripting system, terrain collision system, and capsule-based hitbox/hurtbox system, and I'm at the point where I need to learn animation to proceed. And there's where I'm stuck.

I can successfully import a rigged mesh that animates from Blender into Unity. But I don't know which animation system to use, if I can use both, or anything of the sort, because from my perspective my requirements are mutually exclusive to them both:

1. I need animations to be independent of, and subservient to, character state.
2. I need to be able to control both the current frame and playback speed of animations.
3. I need to be able to apply an animation to any character whether they have the matching bones or not (i.e. animations for being thrown would be part of the throwing character, but be used by the character being thrown).
4. I believe I need to use blend trees for different walking/falling animations.
5. It would be nice to be able to control whether slope conformation is on or off for a character's left leg, right leg, or whole body.

I know there are features like state machines and retargetting available, but they all seem to have something that blocks what I want to do as much as they enable something I believe I need. For example, retargetting looks like it might work for #3, but it claims to only work for humanoids and doesn't have any indication that it allows real-time "here's an animation, play it".

Basically at this point I need a sort of "reality check" as to how I should be going about this. I'm faily sure I have at least one assumption wrong and it's stunting my thought process. Any advice would be nice.

(Yes, I know there's a prebuilt fighting game engine out there, but that's for traditional fighting games, which I don't care for.)