Insulation works merely trapping air. Foam insulation isn’t as effective as fiberglass, because the fiberglass traps more air. Think of the huge difference in a house if you put in a saran wrap type storm window around your existing window. That 3"-4" of open air space between the saran wrap and your window acts as insulation. Double pane storm windows trap air between 2 panes of glass to give you extra insulation. That floor should trap air in a similar way.
For fastening to it, you can use a bolt and a washer that will go through the larger holes, then slide it slightly forwards or backwards, and the narrower part will hold it down, tighten it and it will be secure. I would put the hold down bolts where I wanted to secure things to, because the bolts will come up past the plywood. Perhaps add runners with counter sunk holes for the nuts, to then attach your furniture too. Another option might be to use toggle bolts (nuts), so you’re screwing down from the top, so they can be flush with the floor once tightened. Eye hooks screwed in from the top could also be handy. In short there are many ways to utilize those marvelous brackets, and all without the need to drill holes through your floor.
"Be creative & recycle, reuse, & repurpose." ~ The Camper Van Man