>Can anyone tell me the origins of the plant name 
>
>        P.heterophylla Papalo
>
>I'd like to know if the "Papalo" is an accepted varietal name
No, it is neither accepted nor valid (yet?).
> and, if possible, 
>whether anyone knows any retail or wholesale list or any journal which has 
>published the name.
No idea.
>  Do we know who assigned the name
I cannot speak for others but I do not know that.
>, who first grew or found such a plant?
dto.
>  Even if it is an improper name
It definitely seems so.
> I'd still like all the info available on the name origin.
My tip is: bogus name of horticultural origin, perhaps geography-related.
>  Finally, do we know why the name was assigned
v.s.
 
>(i.e. presumably the person who named it though the plant deserved the name).
Or this person wanted to sell a few extra clones to all those who already
grow the old, boring _P.heterophylla_. ;-)
 
>So is it a reference to a location (Papalo where?)
My tip: Mexico, where _P.heterophylla_ is one of the more widespread and
quite variable species.
>or is it a reference to 
>particular features that differ from the type form and, if so, how does it 
>differ?
It may cost a few cents more than "ordinary" _P.heterophylla_.
Kind regards
Jan