I empathize with how troubling all the uncertainty can be. But if it were me, I would be very hesitant to pay out-of-pocket for a DaTscan.
Clinical diagnoses by an MDS is very accurate--as accurate or more so than a DaTscan. For example, my PD symptoms are quite mild--a couple weeks ago, I participated in a presentation to first year med students about YOPD. Only about a third of the group was able to see that I had any symptoms, in spite of having just seen an MDS give me a neurological exam and being told (by both of us) exactly what to look for.
Yet, three years earlier, when my symptoms were even less apparent, my MDS was able to diagnose PD in about 15 minutes. Someone who sees PD patients day-in, day-out, with all sorts of different symptoms, gets very good at spotting even early, mild PD. If your MDS isn't seeing it, that's worth really thinking about.
I was offered a DaTscan at diagnoses, and my MDS said they would push the insurance to pay for it. I told him if he was sure of the diagnoses without the scan, that was good enough for me. As it happens, I had a DaTscan this summer as part of screening for a clinical trial (no cost to me)--the results clearly showed PD.
The odds are very high you will spend a lot of money to get the scan, and still have no answers. If a PD specialist can't give you a clinical diagnoses at this point, it seems unlikely medical treatment for PD will do you much good either--there are NO proven disease-modifying medications available.
Exercise and good diet are always good approaches, PD or not. So is financial planning, esp. as PD can mean earlier retirement than planned. If it were me, I'd take the money I was going to spend on DaTscan and put it in my 401k.