Thanks for such a great answer, PandaMunich - I wanted to wait until we had had finalised everything before I responded, and we more or less have, now...
My son and I were accepted into the Austrian public health system, and now after the 6 month waiting period (a few days ago), we are full members. In the meantime, he has had an official assessment that his disability level is at 70%. We have just applied for a disability card for him.
Because of this assessment, we get a higher rate of family payment (Erhoehte familienbeihilfe) for him - about 450E or so a month I think, a big bonus considering we didn't plan for it, and only applied for the basic level because they said we should. Also, the disability means that even though over 21 (he's 22) he is still classed as my dependent, so he's added to my public health insurance for free from next month.
The problem with my wife just won't go away, but I guess we can deal with it. After about 4 months and sending 3 letters to the Deutche Rentenversicherung, they did indeed come back a couple of weeks ago, and said she is not pflichtversichert and that there is no deduction from her pension, but they recognised there was a problem and suggested in the same letter that she should apply for German private health insurance. I gave that letter to the Austrians, and they said, well then, she should get private insurance - as long as that is an option, we won't cover her.
So we seem to have maybe 2 or 3 options left to us regarding her cover.
First, after going to the doctor here, we found that they are cheaper here in Austria than we expected! A visit for my wife was just 40E as a private patient, and then just 10E for her single medication for a month. As I and my son are in the Austrian public system now, we have started the 360-day membership countdown that we must have to be able to transfer into the German public system when we move there. Assuming we move to Germany soon after that, my wife just needs to get her monthly prescription until then, that is, for the next year. Although she's 67 (she was a cradle-snatcher, I'm younger) she's pretty healthy, so barring accidents we should make it through the next year.
We had Cigna insurance that we took out very late last year when we arrived here in Austria, which was a requirement to get Permanent Residence. Looking back on it, I'm pretty sure the cover was not actually enough for the local official requirements but still it was 600E a month, and I think the local authorities simply were kind, and decided "close enough", as we were both friendly and very respectful to them, and asked sincerely for their help.
We've just cancelled that private health cover now (runs out in 3 days time) - the deductible was such that we'd never have used it except for something fairly serious and there was a biggish co-pay too. I think there really shouldn't have been either, to meet the local residence requirements, but that they were lenient with us. But, it got us the residence permits so I don't begrudge the 3600E for the 6 months for the 3 of us at all - it could have been much higher. During that 6 month period we also had to pay 110E and 165E each month for the Austrian public health system for my son and I respectively, as this was the 6 month waiting period for this system that we had to serve. Now, the total cost for him and me is 165E a month combined.
So compared to the last 6 months, we're now lots better off (by more than 1000E a month after adding the family payment and stopping the private cover) but we still have the issue of my wife's long-term cover. The KKH (her old fund (she was last with them in 1985) where she must re-apply, and TK who we also asked said they can't insure her as she lives in Austria but that if she lived in Germany TK says that she would be covered under a different section (3, I think) of the SGB and could apply as a voluntary member. She only has a combined German and Australian pension of about 650E a month so I suspect (at 67) the cost of private insurance would not be a good idea. KKH didn't say the same directly, even though we specifically asked them "What if she lived in Germany, can she be covered then?", but they didn't deny that she would be entitled if living in Germany, but simply repeated that as long as she lived in Austria, they couldn't cover her.
I suppose she could move to Germany before we all leave Austria - our daughter has just separated from her husband and moved to Europe a few months ago, and recently started teaching in Berlin so she could go there if she needed to. Or as a last resort to see if Germany and Austria could work out some sort of public cover we could go to the EU Solvit people, but that may not be a serious chance. I think we'll just hang on for the next year and hope she doesn't get sick - her medical costs here will be very low for that time, and then once we actually move, by all accounts she'll be able to get public cover in Germany. Hopefully!
My sincere thanks again to PandaMunich for such thoughtful, and helpful responses!