It’s not. You keep insisting that ^D doesn’t send EOF and yet:

$ stty -a | grep eof
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>;
$ man stty |grep -A1 eof |head -n2
       eof CHAR
              CHAR will send an end of file (terminate the input)

^D is the EOF character. The thing is that in C every line of a text file must be terminated by a new-line. And so, when you end a file with ^D without a return, you get funky results.

If you go with adding a passphrase to the drive keep in mind that if it’s a unique one you may end up forgetting it since you won’t normally be using it. Even if you set it to the same passphrase as root partition, if you ever change passphrase for root you might forget to change home passphrase.

I would probably just make a physical copy of the key file. It’s just 32 bytes (no, larger key file doesn’t make things any more secure) so you can hexdump -C it and copy the key on a piece of paper.

Mint is fine. Rather than changing distros, rather keep using it and configuring it the way you want it. For the most part, GNU/Linux is GNU/Linux is GNU/Linux and many popular distributions are largely the same.