Ok, I've been through the Novi herbs and they contain all the herbs from Character Law and RM Companion I.
Essence Companion has no magical herbs, only herbal catalysts, though the descriptions of these mundane herbs is useful.
RMFRP Gamemaster Law's herb list is simply the same one from RM2 Character Law. Nothing was changed other then tobacco and alcohol were added.
Thanks for the tip on C&T III
Hands of the Healer supposedly contains all the herbs from all MERP books, and also many that also appear in Character Law.
However, in my original master listing (which is similar to Balhirath's), there are many herbs that I've not yet found, a few examples:
Gorfon
Mu-jy-Mirenna
Klandun
Bragolith
Chap Beech Nut
Dextros
Henuial
Kirtir
Muilfana
Numenelos
Plaranata
Sarah-P-H-Head
Strygont
Taynaga
Adrela
Some of these appear in HoTH, but not all, so I suspect there must be another source I'm missing here.
I recall a couple herbs in Spell User's Companion, I think. Are there any in Arms Companion?