No, making a Magic Memory Stick does NOT format it. From what I understood, all it really does is make a few data files for un-bricking a PSP, using the Pandora battery. I updated from 3.40 OE-A to 5.00 M33 using Pandora and MMS, and I didn't need to format my memory stick.
You're much better off using custom firmware, than downgrading. You get new features and stuff, along with being able to use homebrew.
No, newer UMDs cannot be played on lower firmwares, as far as I know. That's why you update to a custom firmware, which CAN play newer UMDs. (Just about all UMDs have Firmware requirements, to play them.)
I followed a couple guides on how to create a Pandora Battery and MMS to update to 5.00 M33, but I can't seem to find them again =/
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