Can't boot into safe mode in macOS El Capitan on 2008 Mac Pro

Well there is your problem. Linux addresses the EFI firmware differently than Mac OS.

A 2008 Mac doesn't support internet recovery. And while you are on the Linux side of things it won't recognize the startup sequence properly, especially if the CMOS battery or capacitor is getting old. So the Option key booting failing probably is indicative of a logicboard aging issue.

I would reach out to Mint users to see what they suggest.

Are you trying to get to the black background, white character screen?

That's command-S.

Are you trying to boot where kernel extensions are turned off?

That's the Shift key.