New ones don’t seem to be available any more, you probably need to go refurbished. Last year some third-party sellers still had new units available. Replacing the battery is always an option.

It doesn’t look like Apple will be going back to something smaller than the 17e. Unless you count the upcoming foldable, which while shorter than the mini, however is also wider, thicker and heavier.

Unlikely. I'm hoping when the foldable models come out that the folded version is a decent form factor. And then I can just not bother unfolding it.