There's no solution for the inverted screen yet, unfortunately. There are a bunch of hardware variants for this player and we don't have a way to autodetect the right LCD settings, assuming it's possible at all.
I have some patches on Gerrit which allow you to fix this using settings once booted into Rockbox (which I should probably merge), but it isn't ideal. There's no good solution for the bootloader, other than providing different copies of the bootloader that have the right screen configuration hardcoded, and you'd just have to try installing them one by one to find out which one works.
The method you followed on reddit appears to be installing the hosted port. That should work fine as long as the stock firmware can drive the display correctly, and the instructions for installing that are on the wiki in the "Hosted Port" section:
https://www.rockbox.org/wiki/AIGOErosQKIf I understand correctly, you unzipped the hosted version of rockbox to one SD card, then switched to another SD card and now you can't boot rockbox. In that case you just need to unzip rockbox on the new SD card. The hosted port should work with non-FAT32 filesystems.