When I saw your post yesterday, I remembered having read recently, somewhere in the 'Net, the ingredients of Tamiya's retarder.  But I can't remember where it is.  I wanted to link it into my reply for you.  It wasn't at Tamiya USA's site.  I may stumble across it again.

I can't remember what the compound is, but naturally enough, it's less volatile than water and so, slows down the drying time.   Might have been glycol, glycerine-I just can't remember.  But I think we have the correct instincts on this.

Yeah, in the absence of knowing, testing is the next best thing.