not sure it is possible (but would be great to have), I go to animate mode to mess around.. then just delete all keys if any, and bones reset to their setup mode pose..

You can try to search for a backup in your backup folder (you can find the directory in the options menu of Spine). Once you found it you need to import the project inside your messed up project, a new skeleton will be added in the list. To avoid confusion deactivate the visibility of your first skeleton, then filter to bones in the tree and expand the root so all the bones are visible, select them all and select all the constraints as well if you have any. With coordinates set to world press ctrl+C.

Then deactivate your backup skeleton, maybe collapse it and open the other one. Select all the bones and ik as you did for the other one then paste the position by pressing Ctrl+V.

Note that there need to be the same number of bones selected and it's best if you didn't change names or hierarchy for this to not end in something ugly.

If you did change bone names of hierarchy then use the backup skeleton to confront differences, iyou can still copy-paste one bone at time.

The question is: what is the point in time to reset to? You will have to manually specify this, at which point you may as well save out a new .spine file.

That really isn't the question though based on ops question. This is something that should already exist and would be a cakewalk to implement. Simply add a "temp pose" button it is that easy just seems like it isnt a pripority to development or management.

You're right, it isn't a priority for development because we don't see a use case for it. We already have a temp pose mode: it's Animate mode. If you really want to move things around in Setup pose you can, but you'll need to use undo to get back to what you had before.