You downgrade via service mode again (the same process as when you installed modded 3.41, but with a 3.15 OFW instead). Then you exit service mode. Before updating to 3.55 I suggest that you go to service mode and use the 3rd option "Restore File System". This could restore your BD playback, however it is not guaranteed. Once this is done, update to CFW.
And yes, NAND PS3's are easy to brick with Linux, that's why I haven't tried to install it on mine even though I want to. NOR ones don't have issues with OtherOS++ at all, but the PS3 models with BC, 4 USB ports and card reader are all NANDs
PS3 Slim CECH-3004A 160GB
PS2 Slim SCPH-70004 - FMCB 1.8b