You should install bootloader before problems with internal flash.
I agree, but this bootloader boots from SD card only if it finds internal storage faulty.
It work only if rockbox can't mount internal storage.
So I'm not sure if I should use this one, or wait for binary build for "SD-card only" bootloader as you suggested.
Yes, but better wait until someone provide tested binary build.
In your case: if rockbox still loading and you can see sd card - you can load new rockbox from sd card by hands.
Extract "rockbox.zip" to sd card. Enter to ".rockbox" dir and copy "rockbox.sansa" to root dir. Then boot rockbox and select Files > rockbox on sd card.
I already returned the player to the seller for exchange, but I remember it was impossible to even access audio files for playback. It returned Plugin error and returned to main menu. The stub I pasted into my last post is taken from dmesg.