Which mitzvos did Adam have before eating from the tree?
1 - Not to eat from the tree.
2 - (the above +) to have children.
8 - (the above +) 6 mitvos
9 - (the above +) tzedoko
Your question already mentions Rambam's Hilchot Melachim 9:1 as a source that Adam had at least six negative commandments
Radak derives each of these six commandments from the words in Bereshit 2:16 "And the LORD God commanded the man, saying: Of every tree of the garden you are free to eat" (see also Sanhedrin 56b). Therefore by definition these six commandments were before Adam ate from the tree.
The seventh and eights negative commandments are
Beyond that we have at least two positive commandments appearing before the prohibition to eat from the tree
On the verse "Keep [Gan Eden] and tend it" (2:15), various commentators (see here on MY) have suggested this could refer to physical labor, Torah study or sacrifices, thus making the total number much larger.
As confirmation, I asked R Binyamin Tabadi who agreed with the above. He offered three additional mitzvot (as found in the writings of R Nissim Gaon), suggesting they were given from the beginning since Adam had a relation with God "from day one".
I have since then found a source that listed these, as well as the full list of the 30 commandments of Bnei Noach (based on Chulin 92a, see here on MY). Some of these 30 could possibly have been given to Adam before he ate the forbidden fruit although I don't think there is a source for that.