I took a ferry near Puget Sound on a road trip in 2018 - from Clinton to Mukilteo - is it Possession Sound? Genuinely just thought it was a regular sea crossing - I suppose looking again at the map that area is more like a river than a regular seaway.

The rain held off for the most part - although the greenery was spectacular compared to the other (?rain shadow) side of the Cascades. Lovely part of the world. Good you don't get the rust problem - same here in Scotland. In the sea lochs it isn't a problem - on the coast and islands on the other hand...... it's hellish!

Any form of salt accelerates corrosion in any form of steel, iron, well, pretty much anything (yeah, including certain types of 'Stainless Steel').

If you live next to an ocean or sea you'll know all about it - hint: DON'T buy a car or machinery from an island area.

Salt mixed with water and oxygen will simply accelerate corrosion beyond belief - it accelerates the transfer of the electrons from pure iron to form iron oxide. For an example take a look at ladder irons on a tidal quay, pier or shore structure. The rungs permanently underwater will be OK(ish), the rungs halfway up will be pretty badly corroded, the rungs above the water will be utterly destroyed. It needs all three elements to make rust - iron, oxygen, sodium chloride.

Where all three are present in sufficient quantities they can literally reduce an entire vehicle to oxidised nothingness within a decade. This includes entire engine blocks etc.

Roads are optional.

A general rule of thumb with insulation - either board or glasswool roll - is there is incredibly little waste. Just cut to size and put it in the gaps as best you can. I'm assuming you're going to be covering with a vapour barrier and plasterboard / drywall in any case? It doesn't matter a damn how the insulation looks - as long as it's full coverage with as close to no gaps as possible.

If you get rolls of insulation for the required area just cut to the required width (in your case 22") with a saw - while it's still in the roll. Sheet insulation can be like the worst made jigsaw in the world - who cares? It's going to be hidden in any case. Just slice it all to fit nice and tightly to the studs.