OF / rockbox

can somebody clear things up for? if i install rockbox BOT rockbox and OF are on the player? only that  rockbox once installed  is the default choice for the player always ? and i activate (switch) between them if i hold LEFT while booting (back to OF ) ?

and second :

what do i do if the rockbox installer will not work anymore cause files (like the database files and config files) got corrupted or deleted? how do i install/uninstall rockbox now ? is there a manual method as for the OF

Yes you have both firmwares and can chose which one during boot. You can uninstall rockbox from inside the sandisk firmware, check the manual.