My only suggestion (and not quite responsive to your question) is to rent a car @ Pto. Montt for these travels, which will make it much more convenient. I've been down the region several times over the past 15 years, although my last visit was probably 3 years ago.
Given the relatively short distances, I wouldn't be surprised if colectivos ran between Pto. Varas, Frutillar, and Pto. Octay. I've seen small buses (more like larger vans) in the area but haven't really paid much attention because I've always rented a car.
(Note, the colectivo chileno is a little taxi-like car that runs along a specified route "when it shows up" for a fixed fare. There likely isn't room for much of any luggage.)
Out to Petrohué, I've only noticed tour-type vans, but maybe there are other services, too, and again, I've not paid close attention.
Note that the road on the east side of LLanquihue, unless it has been improved in the past decade, is a road you shouldn't drive on, nor should you expect any transit to ride on. I drove it once in a rental car. I am very grateful that neither myself, nor the car, suffered any damage.