ok firstly you must have 3.40 oe. secondly you must have a decrypted paf.prx from 3.40 oe. third you must have the gui app or a hex editor. if your trying to use this gui with the 3.51 it will NOT work as previously stated. ill gie you the benefit of the doubt and assume your on 3.40 oe. one other thing before i help - NEVER mod pspbtcnf.txt except for vsh.prx loading - it is the only file that has to be redirected so 2 copies can load. also your reference that you found is valid because dark_alex patches the paths in memory so the pspbtcnf file can remain untouched to preserve the encryption i believe. now then - lets begin:
you have tried an original encrypted paf.prx just to make sure your source files are not corrupt. if not do this now.
if this allows you to boot wee may proceed.
next try an original decrypted paf.prx overwriting the old one in nodule directory (not Module).
again see if it boots. if it does THEN and only then can you use the gui to hex edit the file.
if you get this far you can't really go wrong provided the gui actually edits the file correctly but if you follow these instructions you can be sure that the gui will be the problem if it fails when you do your first hex edit.