I think you're right in that the water from all of these sources should contain little-or-no scale chemicals - but I'd be reluctant to use it for this purpose, purely because of the risk of bacteria/viruses that could be present and would get breathed directly into your wife's lungs.
A couple of better sources come to mind:
1) A standard filter-jug. Having used one of these for many years, I can verify that scale build-up in the kettle is negligible. Clearly the filter is absorbing the scale very effectively.
2) Water softener. I assume you don't have one, but if you have a friend or neighbour who has one, maybe you could ask them to let you have one or two litres per week, given that it's for health purposes? You would need to keep the stock refrigerated though - otherwise you'll have the same potential for contamination that I mentioned at the beginning.