IMO, the stuff needs to be fixed .... all of it. Get all your ducks in a row, schedule a time to shut down the water long enough for you to install a new shutoff, and do it. If they tell you that they can't be without water for even a short period then walk. One thing to consider, if you are jerking around on that rusted ball valve and something gives in a not so good way, they will be without water for a hell of a lot longer than a planned outage.
Jim, I use PB Blaster as well. Best penetrating bolt breaker/lubricant out there.