Pros and cons to all of the designs presented.
If departure angle is a concern then anything connected to the stock hitch is going to get hung up. If it's just to keep the spare to the trail head and then moving it inside the truck then I'd say it doesn't matter. But if you're planning to use this as a mount to permanently keep the spare there then you're going to want to consider the departure angle issues. The 4th gen ass is very low so it already gets hung up on a lot of stuff so just keep that in perspective.
The last design w/ the tire mounted to the driver side is actually not that bad. The tire mount itself sits level w/ the rear bumper so it's definitely higher than your hitch which helps on the departure angle (not sure about the bolted hitch though - defeats the purpose but I see the need for the hitch so go figure).
The only concern on using that rear crossmember/bumper is that it's a very thin piece of metal. Grant it it's squared and not a c-channel so I guess it's ok but I wouldn't want that to be back there long term. I fear the fatigue on that rear crossmember might become an issue (don't quote me cuz I'm not a structural or metallurgical engineer).
If you really want to do it right I'd say just pony up for a rear bumper. I know costs is an issue (we've been been there and done that - you've done it twice with 2 4runners so I know your wallet's gotta be hurtin a lot more than any of us here!).
For an interim deal I think your concept works. I've see plenty of people do something similar. BruceTS (Bruce) has done something similar too. The only difference is that he had his tire tilt down away from the 4runner and not to the side (i.e. like a tail gate but instead a tire gate). I thought he made a thread with photos but I can't find em.
As long as it doesn't come off I think you're in good shape!