This project overall is excellent and IMO deserves wider recognition. I almost want to create a new game just to use it :)
To answers your questions:
1) A few words of description (maybe onmouseover titles?) in UI would help. For example, I was initially confused by the fact that buttons on animation preview pages do not change their images when I pick different hero (i.e. I thought that buttons themselves were preview while in fact you must press them to change preview animation).
2) Export options are just fine. Since target audience is programmers, it is relatively easy to convert between spritesheet and list-of-sprites, so IMO you could even get away with fewer options. If you still want to add more, one idea is to export each animation as a single animated png (or gif).
3) As I said, UI is a little bit confusing at first, but honestly it is quite a minor hurdle compared to the value of the project.
4) The only real improvement I see for this project is GIVE ME MORE :) In particular, I think melee attacks are obvious missing element
5) They are nice as "inspiration generators", basically a more consistent version of generative AI. As a dev, I would also be interested in exporting palettes as e.g. JSON to facilitate programmatic recoloring. Also, some 'effect applied' palettes like e.g. burning, frozen, etc. might be nice