Yes it will make clean soft water from river water, but only for a few hours. Then the inlet pre-filters will clog and it will shutdown.
Making fresh water with a fresh water membrane tests better than any city water. With a salt water membrane it tests about the same as city water.Just how good is the water it produces.
Yes I think your right as the more I learn the more complicated this start up becomes.How are you going to power it?
If it needs a new membrane, likely, they should be pickled annually whether they are used or not, add that to the mix.
With the potential cost and effort of getting this up and running, a good multi stage filtration system may serve your needs just as well since all you are wanting to do is clean up fresh water.