Community: Energy efficiency and durability

I have 2005 Simonton Builder windows -- dual pane, E-coated, Energy Start etc.

The builder put 2 ,side by side, single hung windows in a small bedroom. That's a 6' x 6' opening. The bedroom has 3 exterior walls.

The room gets cold. The window seals leak air, and I've tried everything I can to plug the leaks.

I want to try the 3M window shrink film. I noticed some people make a wood frame and put the plastic on both sides, making a trapped air space. Is it worth the extra trouble to to have 2 films versus one?

I have decided to write an article on the topic of Cut-and-Cobble Insulation. Even though I sometimes scoff at the technique, calling it an unprofessional method beloved by homeowners, I must admit that I have used cut-and-cobble myself. Sometimes, the method makes sense.

For those who don't know, the term refers to the insertion of strips of rigid foam between studs or rafters. Each rectangle of rigid foam is sealed at its perimeter with caulk or canned spray foam.

So I'm asking for help.

A Mitsubishi Rep. had posted a load run comparing there two systems stating that Hyper Heat Inverter would cost twice the cost of geothermal systems including installation.

I had an energy audit yesterday, and they claimed I could get a 5 year payback by adding more attic insulation. I found a payback calculation online, and that came back with 30 years. I want to do the right thing, but it also needs to make financial sense. Anyone have experience with the payback calculation? I put my specifics in the "more explanation" section. Did I do it correctly? Hoping to find someone smarter than me (that part won't be hard) to help. Thanks.
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Current insulation is R30 – fiberglass