Don’t let this stress you. You are overthinking this, and worrying to much.
Public transit does not just evaporate. The locals need to get around, so when a railway is closed and alternative bus service is put in place. The people of Stresa aren’t just told: Sorry, you will need to stay at home this summer.
That not all details for a schedule in September are not yet in place is normal. So what you should do is relax. There will be a way to get from Luzern to Stresa that does not involve a rental car with a huge one way drop off fee.
One interesting route you have is this:
Luzern to Locarno, and then boat to Stresa. You would have to plan around the times of the boats, but you could spend some time in Locarno as well. Would make for an interesting day.
But going by train via the usual route (with maybe partially a substitute bus) will also be possible. You can count on that. So stop worrying about that.