The analogy with having the right "bit" for a screwdriver is a great analogy by Stefan! I have a screwdriver set that has 25 variation of bits and still may not have the newest strange one and then there are at least 10 that have never been used. This seems quite analogous to cinelerra-gg.
1) Stability is pretty good in my opinion.
changing the mode to executable (chmod +x CinGG-202010331-x86_64.AppImage) and then running it.
4) Since I know next to nothing about animation, the opinion of Stefan is most valuable. There is a very short description in section 17.5 of the manual describing Festival (simple animation) that was written over 20 years ago so is most likely irrelevant to you.