Personally, I'd skip the rino-liner. It adds a bunch of weight to the boat. If you're going to put a floor in if,why bother with what the bottom like like any way.
If it were my boat, I'd not anchor the deck to the floor at all. I'd build a frame for support, and screwing the deck to. Use 2x2's or 2x4's or make depending on how high you want the floor. Then make a template from paper or cardboard to transfer to marine grade plywood to cut the shape of the deck. Cover with marine grade or indoor/outdoor carpet depending on how much money you want to spend.? It looks like it had a deck on the front at one time. Support that frame to the sides of the boat with riveted aluminum to the height you want. I would do it level with the front bench, the run more marine grade plywood covered in carpet over that.
I like the open middle. Maybe get two or three seat peristal bases in the floor. One towards the front, one I the middle, and one in back. Then you can have a kid in each spot, but if there is dad, grandpa, and maybe one kid, then just use the middle pedistal base.
I would also use the space under the front deck for storage of anchor, life jackets, etc. you can buy hatches from cabelas or scheels, but you can make your own for a lot cheaper. They now sell bow and nav lights that are battery operated, and the clamp to the boat. I used em on my last boat and they were fine. For interrior lighting, get some battery operated LED's. That have small narrow strips that stick to the boat. They're fan-feakin-tastic! Hopefully this helps!
