I'll come back later with more, but...
Can I run 16 and 19 parallel- in other words run 16 separately from 19 until 19 is up and going?
Yes.
Basically. Leave MC16 as it is. But, before you do anything, make a Library Backup. I'm not 100% sure of the procedure in MC16 anymore, but make a Library Backup. Probably in the old Library Manager.
Then, install MC19. It will set itself up separately.
Then, you can restore your Library Backup from MC16 into MC19 using
the new Library Manager.
But, you can have different major versions of MC installed side-by-side way back to MC12 I think. Certainly it will work fine with MC16. You shouldn't run both at the same time, but you can have them both installed and use them independently.