I'm a little late to this party, but Uber has a "Car Seat" option with 4 or 6 passengers if you scroll down when picking what type of vehicle you want.

consider a rider safe vest. In a situation where you are sitting next to the 2 y.o. helping them stay in a good position it should be ok.

I would probably get something like a mifold or a bubblebum if I really couldn't bring a carseat (although I'm not sure why you can't).

My logic would be that being a few lbs under the weight limit would be a lot safer than not being restrained at all, or being several feet below the height limit.