Ok everything is working and looking as expected. I'm sure some of what I did was a bit reckless but as there was nothing (yet) of import on this computer, and I had a Windows recovery USB stick at the ready, I really didn't care too much.
gohlip - a HUGE thanks for all of your assistance. You've been invaluable in helping me out here!
I still don't know what the cause of my problems were but it may have been the limit on the number of EFI bootloaders my bios allows as you and others suggested, some problems in naming conventions, or a host of other things.
Ultimately I had quite a few directories in /boot/efi/EFI due to my many attempts to get things going. Using efibootmgr -b (entrynumber) -B I first deleted what I thought was an unneeded entry, followed by physically removing the directory itself from /boot/efi/EFI.
I had two manjaro EFI directories. One was named Manjaro, the other "manjaro" (quotes included)! I planned on removing both of them and re-running grub-install, but instead removed the Manjaro directory and renamed the "manjaro" directory to manjaro. This seemed to do the trick.
My remaining ubuntu grub was actually just another instance of manjaro - I must have deleted Mint's grub by accident at some point.
At this point, my bios now only shows two choices for bootloader - Windows and manjaro. That's good indeed.
One thing I noticed was that, with all of these options, the bios seemed to re-order the bootloaders, despite running efibootmgr -o 0002,0001,0005 etc. So it's possible that all the work we did last night was somehow preempted by the bios. Again this may be due to the sheer number of entries. At one point I had:
2x fake manjaros
1x Windows recovery
I think the next question is what happens when I update the kernel ;-D
Again, appreciate the help all. Hope my steps and missteps help someone out.