If you go with the defaults, they might be. But if you manually define the letter for your external drive, it will keep it forever. (I have my external drive set to X. I’m not sure if Windows would respect that assignment if I had plugged in 19 other drives, but that is never going to happen.)

Not while it's mounted. This is akin to complaining that on Linux if you unplug a flash drive and plug in a different one that second drive could "steal" /mnt/sdb1 or whatever.

You can fix the drive letter assignments at any time if they become a problem, or use a directory as a mount point if that's less troublesome. (Win-R, diskmgmt.msc)

I remember vividly when a user couldn't access his smb drive from Windows because both his printer and also the computer's case came with one of these multi-cardreaders with n slots and the drive letters collided. That's when I learned that smb drive letters don't even come from the "global" pool of drive letters, because, and this is obvious in hindsight, they are a per-user affair (credentials and all that).

I think the concept of drive letters is flawed.

Only if the actual "drive letter" assigned to the drive is the special value for "auto".

Otherwise, the drive letter is allocated statically and won't be used by another volume.

You can't work anymore only if you are incurious and unable to google a simple solution - assign a different drive letter with the disk management program.