final Being able to RAM dump from fastboot mode is guarded by a check for the device being unlocked. If they're able to do it without being unlocked, that's an exploit, even if it's a trivial one. If the flaw being used in fastboot mode was known, it would be fixed in a firmware update.
There shouldn't be relevant data left in memory after a normal reboot to fastboot mode, but data can persist through reboot from a kernel crash or similar situations. Overheating is also known to trigger an emergency reboot.