PSGroove is a modified Rockbox Firmware. It disable the USB HDD access functionality to create USB Hub emulation for PS3 jailbreaking.
If you replace the original rockbox firmware with a PSGroove patched one, then you will need to use the disk mode to connect the device to a computer and access the stored files again.
Disk mode is an hardware mode from Apple (and other devices?) to access the files on the device.
What I recommend :
Don't replace the Rockbox firmware. (so, when you boot your device, the official rockbox firmware is loaded and you can use it as a fully functional audio player device)
instead, create a folder at the root of your device (ex. PS3)
in that folder, place all the PSGroove firmware you may need and rename the files to something more recognizable (hermesv4b.ipod, PL3+PSN.ipod, etc.)
Then, when you want to jailbreak your console, just use the file browser of the rockbox firmware, navigate to the PS3 folder and select the firmware you want to reload into.
When you want to return to official rockbox firmware, either reset the device or select the .rockbox/rockbox.ipod file to reboot into.
For more detail, check the tutorial I made on WikiTemp :
http://wiki.gbatemp.net/wiki/PSGroove_Rockbox
Installation, part3
About your questions :
Rockbox can be uninstalled (but you would never want to return to Apple!). Use the Rockbox installer, there's a menu to uninstall rockbox.
Note that the original Apple firmware is not deleted, it's just a multi-boot device. hold nothing -> rockbox, hold menu+select -> disk mode, switchON and put the lock switch to Lock state -> load the Apple firmware, etc.
There's a way to modify that key combination, and decide to boot Apple firmware by default, and load rockbox with a key combination.
About the menu loader you saw, it is safe to use.
It just replace the official firmware by a menu firmware, and use a config file to link the menu content to actual firmware files. When you select one from the list, it actually reboot into the selected firmware.
I don't like that menu, as I'm using the device as my main audio player, I like the fact that it auto-resume the last played song when I power ON my iPod.
I prefer my method of rebooting into the PSGroove firmware manually when needed.