Hi
No. Often discussed in this board. For example here
It’s open core. The used encryption library is open source
See description here:
There is no donation key for the mobile apps. You pay for them a onetime fee.
If you want Darkmode for the desktop app you need a donation key (means you support the Open source desktop app with a donation)
Of course. Feel free to make backups and store them where you want. But there has to be a Masterkey file in the root of your vault folder. But if you want to achieve this „extra“ security by moving this file out of the vault then the answer is: no, that’s not possible. And btw would not bring any extra security because it does not contain unencrypted data or makes the file encryption weak in any way.