Please give me the option in global options to set "Multiple Files Page Name..." as the hard default PDF option in Map Series. Brownie points if I can completely disable the other options.

No - I don't want a preset, I want something that is a hard setting that I can rely on to be the same every time I open Pro and click share layout as pdf. I'll be happy if you remove the Single File option altogether and I have to merge my pdfs using Python or something else.
This random stupid behaviour that you have to double check every single !! time is getting old.