Winterizing summer cottage

Does anyone here have experience winterizing a summer cottage for
year-round use? Is it feasible to put fiberglass insulation on top of
plaster (inside the house) and sheetrock on top of that? Not an elegant solution, but workable?

Depends on the type of construction. With most types, I think your plan would cause condensation to build up between the fiberglass and the interior side of the original plaster walls. You probably should have a vapor barrier installed on the outward facing side of the plaster wall framing or replace the plaster with sheetrock and install the vapor barrier then.

plus, you would need to re frame and trim every window and door.  Why not have insulation blown in?  Probably cheaper.

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