Non combustible decking options

It would be best to use two. If you make sure your deck boards are cupped down you would think that only one screw would be fine, to stop the board from cupping up in the center. I have seen wood cup in some extremely odd ways so this is not always the case. Two per board/joist intersection will ensure you do not need to mess with it in the future. Or you can always just use one and wait for the weird boards to cup :) I prefer low maintenance and the screws shouldn't be too pricey. Good luck!

I agree with Ecnerwal entirely. But, absolutely screw the deck boards down & don't use nails, unless you like them popping up & ripping your foot or catching a shoe or high heel. Screwing the boards down is structural, for mostly just the boards...it spreads the stress instead of snapping a board from a fatboy standing between joists.

Definitely, use the tar-paper & bravo on doing it in the past...almost no-one around me does it & laughs at me when I do it or point out it's missing. Their decks last maaaybeee 15-years & mine last easily 30+.

Go with stainless steel screws! Square-Drive like the old screws is fine, but mostly Star-Head is all I find anymore. I think chamfering the bottoms will only trap more debris on top of the tar-papered joists, this will also cause water to travel to the meat area rather than just the edge. Simply hosing, brooming or putty knifing out the new gaps annually should be greatly successful.