Thank you Jia,
As I mentioned (and quoted) in the original post, H2(g) was indeed not included by Dr. Gustafsson on the converted-to-GWB version of the Minteq dataset, and I wonder why. It is possible to add it and successfully have ACT2 represent the water stability space, so the omission is unclear to me. I have not been able to reach him.
I hope someone else may chime in on adding hydroquinone! I have been only looking at the databases themselves, which provides a lot of information already, but I will check the user guide on this.
Many thanks,
MC