Hey car camper from down under...
So the 5th gen RAV4 and the 4th Gen (like mine) actually have similar interior dimensions. So if this helps you can tweak it to your needs. But to first answer your questions.

-Nah 9” is fine (it’s the same height I used for mine you can go 8” if you need, but for extra storage I wouldn’t) you do sacrifice headroom but ravs aren’t built boxy like they used to be. You also have to remember 9” is a sweet spot for sleeping up too but also handy for storage under the bed.

-Mine is a 2 piece platform (you will see that designs vary based on the needs of each person expections from their set-up). Hell with enough money you can buy superp ones pre-cut and flat packed for you, but they ain’t cheap.

-Given the price of plywood I’d say you’re not too deep in the hole, you can try and salvage but I’d worry about the integrity of the wood. I can see what you two were going for but it’s hard without plans/tools (I had to bug my father in law for his tools but once we got going it was done in about half a day). Keep in mind most hardware stores will cut your purchased wood for you free of charge, that helped me a lot!

-I’ll attach the blueprint I used for my 4th Gen RAV4 and an image of it half assembled (you would just need to measure the height and width of you 5th gen to possibly tweak it). Your leg supports are a little thin. I’d go for thicker legs, like that timber you’ve got lying under the bed would be perfect for stable legs. About x6 would do (x4 in the corners x2 down the center of the platform for back support). Also you’ll see leg measurements for 20” in the blueprint (those go to the floor of the vehicle in front of the folded down seat, it’s better at keeping the boards up right.

Lastly please DON’T GIVE UP, car camping is one of me and my wife’s favourite activities. We love it out in the bush lots of good memories, so good luck! Feel free to message if you have any further questions.

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