Thank you very much for detailed feedback!
Let me try to clarify some points:

1. We can improve spacing between images, right now if you upload multiple it just adds them without extra line space, we can try adding extra line to solve that.
2. Captions are usually alt text which suppose to be short and explanatory for that particular image, that's why personally wouldn't suggest multi line caption.
3. Deleting caption deletes image, this shouldn't happen unless after all caption removal still backspace is pushed, backspace basically attempts to delete whatever the previous item on screen which happens to be image itself.
4. If snippet bringing old editor that's due to snippet having some html code which new editor doesn't support, try resave some of your snippets without any html codes that should work on new editor as well.
5. Line spacing, could you please elaborate or share screenshots of what you wanted to achieve so visually issue is clear for us?