- No idea but why not do that?
- Updates are done OTA but may only contain diffs using the base OS. So they may be broken if you dont flash both slots. Absolutely flash both slots.
- You should wipe that phone clean, then it doesnt matter and just flash 21 directly.
Also, note that outdated phones will mostly still run an outdated and insecure kernel, and have not updatable, vulnerable firmware.
Idk Android is likely somewhat secure, but I wouldnt want to daily drive something different from a (used) Google Pixel with GrapheneOS anymore. I get updates like 4 times a month!