It's safe to say that cutting 100mm PIR can get in the sea
Ugh, the thought of it sends shivers down my spine. Nice idea with the tracksaw. With a dust extractor connected too? If I ever have to do that kind of thing again I'll give this a go. I found cutting to exact size too time consuming so I ended up deliberately undersized by 10mm and then expanding foamed around all the edges later.
Re the overhang, I'd consider making it a little longer if you don't have a compelling reason not too, it will really help keep the front of the building dry. With 365mm I think you'll get splashback from rain hitting the floor, and direct rain in mild wind.