Thanks for sharing your suggestions! FWIW, you can see what we have planned here:
https://waffle.io/EsotericSoftware/spine
1&2) We are finishing up version 3.7 (audio and other things), then version 3.8 will have a curve editor.
3) Separate keying for x/y is planned. You can subscribe to the issue on GitHub and get an email when there is progress.
4) Scaling to flip isn't really a trick. I'm not sure it would be improved with an explicit flip.
5) Note that Spine saves backups periodically and when you save. It uses a numbering system to organize the backup files, but it sounds like you want to save a copy with a meaningful name. Save As does what you want, I think? Maybe you are asking for Save Copy? That would do the same as Save As, except it wouldn't change to the newly saved project.
6) There are many benefits to knowing the hierarchy. It allows for assumptions/optimizations and some hierarchies are not valid for some features. Allowing the hierarchy to change increases the complexity of a number of things, which is why we chose not to allow it. You can simulate multiple reparenting with multiple transform constraints.
