Attachment - bond or rivet? I'd instinctively rather rivet in case I need to remove the floor in future but what are likely issues with this? I have some concern about corrosion/different materials reacting and also the proximity of rivet holes to the edge of the composite floor.

I have thought about doing the floor on my kitcar floor but I would like to make something a little more complex and incorporate a vortex generator into the floor, I'm just waiting for Fusion360 to build in CFD simulation have heard it's on the list

Regarding the stack, I would go for a complete carbon fiber floor with very little if any kevlar, and for the unsupported spans, I would consider adding shapes into the sheet in choice patterns with maybe some Soric in places to stop the sheet resonating, kind of like bead rolls in metal sheeting, I would do this by adding profiles or Soric into position onto my glass table before laying up the carbon stack for the area. 

Davro would know more about these kit cars than me, so his suggestions are probably good. I have built floors for Stohr F1000's, which may be pretty similar. These floors are generally 2 layers of fibreglass with 15mm honeycomb in the middle section and 10mm divinycell in other areas like the rear diffuser section and sides. The areas with no core are 5mm solid fibreglass. I would look closely at that layup and save a bit of weight/thickness by doing it in carbon.