It has been ages since I looked at this one. However "2 byte english characters shows just fine the problem are the one btye ones" sounds like you might have encountered the ever fun "technically it is shiftJIS but actually the 8 bit stuff is not there" that most DS games that do such a thing will do.
If it does not cause memory issues then go with the 2 byte stuff. JWPCE, njstar and most Japanese text editors will have a mode that will default to this, might even be able to coax a normal IME into doing it but eh.
If not then you get to find the encoding and alter accordingly. If it is one the the NFTR fonts then you can add things in easily enough (should have a guide in http://www.romhacking.net/forum/index.php/topic,14708.0.html
, tinke and nftr editor being able to get the job done). If not, and there are no unused 8 bit things you can overwrite, then it becomes rather harder.