At 2, it might also be against the law for him not to be in some kind of carseat!
Honestly, church communities are often so very helpful - I'm not exactly a religious person, but have gotten a lot of support (emotionally & financially) from our local church when I needed it most
If it were me I think I'd go with the EZOn - the extra tether anchor kits don't cost a whole lot so then it would just be a matter of gettig the other adults to let mom have them installed ... to more easily change the same booster & harness straps for every car.
I would think considering this situation that a LapTop might also be an option, depending on his height, but I'm not sure if there are any on eBay at the moment *SIGH* (Worse comes to worst, I have a spare LapTop that hardly gets used now that we sold the vehicle with a lap belt - if there's no other option for this mom I'd be willing to part with it for as much as it costs to ship)
Otherwise, I think the Ride Safer vest might be the only viable option - wish I had experience with it to share with you!