Thanks for trying!
One further confirmatory experiment would be to try dissolving DMT fumarate in some acetone spiked with the equivalent amount of acetic acid to the molar amount of DMT present in the DMT fumarate solubility test material, stir for a bit, remove the liquid and evaporate in a weighing boat. This would demonstrate any solubility that arises from the acetic/fumaric/acetone combo. It's possible that some equilibrium amount of DMT acetate and DMT bifumarate will form - the pKa's are fairly close, so maybe up to 10% as a rough guess - which, if the bifumarate also has appreciable solubility, will lead to up to 20% of the DMT going into solution.
At least evaporation recovers things, and acetic acid is volatile.
Maybe the precipitation can be forced with excess fumaric to a certain extent. After that (and after removing the supernatant), strong evaporation will drive off most of the remaining acetic acid and a quick rinse with fresh acetone will clean up any remaining excess fumaric. Maybe.