Back stab uses its own functions to determine hits and crits, and so doesn't check many of the regular combat functions (to do things like give xp). I'll look at it tonight and see how difficult it will be to make it use combat's normal functions for hits.
I don't know if this is a bug or wad, but when using back stab you get no exp for the crits or hits that you get with it on npcs. I would assume same holds true for assassinate if thats still around.
I won't be changing back stab - its code is commented enough for me to believe that it's a matter of balance and not preference as to why it's done the way it's done. For the record, poison is transfered on a successful hit that deals a certain amount of damage or more.
Reverse stroke is a passive skill checked for by back stab and assassination, so I won't be changing it either for the reasons stated above.