You'll see a variety of techniques used. I think that a preferred method, where there is access, is through bolting with backing plates to distribute the stress. In areas without access, many glass aluminum plates to the underside of stress points. The aluminum plate is drilled and tapped for fasteners.
Coosa is a great material with excellent compression strength. That's why it's used so much as a transom material. If you can through bolt, properly encapsulated, Coosa should be fine for the kinds of applications that you describe. It may be overkill for an entire deck, but there's no question it's lighter than plywood and plenty strong. Many use a lower density foam like divynicel or a honeycomb plastic like nidacore for their decks. You could still build in Coosa pads at stress points where you can throughbolt.
Some builders are trying sipomething new. They're installing solid glass pads and either drilling and tapping into it or useing self tapping screws to install t tops and similar hardware. There is some engineering data that indicates this is quite strong and has the advantage of eliminating the corrosion of dissimilar metals like SS bolts and aluminum plate.
Dirtwheels 222 is one example of this technique in use
https://classicaquasport.com/smf/index.php?topic=13425.0 I'm useing this on my rebuild as well. I'll let you know how it works out.
There are numerous variations of these techniques. Some find a way to tie t top mounts into stringers. It's also a good idea to drill holes oversize and fill with thickened epoxy in order to protect the core from water. Part of the fun of doing a rebuild is figuring this stuff out.
