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The best move would be to simply make sure the existing vapor barrier product is installed properly (flush to ceiling below) and add blown insulation over top to to get the best R value and at the least expense.
The masonry work should be performed prior to repairs to the existing roofing, unless the roof is leaking, whereupon you may need to make intermediate repairs to stop the leaking while the masonry work is performed.
The new ridge vent should have the bug/snow/rain barrier integrated into the venting system. There are probably many other areas in the attic into which "bugs" could enter, but attics do not typically make the best habitats since the temperatures can be extreme and there is no "relatively nearby" source of water for them to survive. The "integrated mesh" is more for helping to prevent wind blown rain and snow from entering the attic space and settling on whatevr lies below.
Hardi™ is a fibre-cement product and the LP product is a chip-board/resin composite. The Hardi™ comes both finished and unfinished, the LP is unfinished and must be painted with a high quality exterior coating that meets the demands and longevity requirements of the region in which it is utilized. Finished Product warranties vary based on the above variables.
For most gensets, simply changing the orifice and a few adjustments will allow the conversion.
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