I've tested my rain water a couple of times where I live. TDS came out very low as well (around 5 ppm, as I remember). If your rainwater is clean, it's close to ideal as a starting point for remineralization if you can collect and store it in a consistent manner. Those are big IF's however, but of course, you can always use rainwater when it's available and RO when not - say, during the summer where precipitation is low and in the winter where it might not be possible due to freezing etc.
It is. Rainwater is naturally distilled so it will only pick up hardness if it is contact with <"limestone containing dust"> etc. Rain water is naturally slightly acidic from the small amount of dissolved CO2.
The dKH test measures alkalinity, so that if the factory emission was sulphur dioxide (SO2) rich it would make the <"rain more acid"> and lower the alkalinity reading.
