While I certainly couldn't advocate enthusiastically launching your vehicle off every speed bump you come across, a shock load such as you describe was definitely taken into consideration during the engineering process for the Curt # C13292 Class III hitch. Reasonable driving wouldn't cause a problem.
The hitch has a 525 lb tongue weight capacity, which will work perfectly for the combined 340 tongue weight load of the wheelchair and carrier.
We have an installation video that you might... view full answer...
The Curt Trailer Hitch Receiver # C94ER you were looking at for your 2021 Buick Envision weighs in at 25lbs.
I included a link to the written instructions of the # C94ER for you to take a look at. view full answer...
The Roadmaster Dual Hitch Receiver Adapter for Tow Bars - 2" Hitches - 2-1/4" Rise/Drop # RM-077-2 does measure 2-1/4 inches from the center of the hitch pin hole on the shank to the center of the hitch pin hole on the receiver.
You definitely will want to move your Thule EasyFold XT # TH903202 bike rack off of your flat towed Buick because this rack is not approved for use on the rear of a flat towed vehicle. That means if something were to happen to your rack when it was on a flat towed... view full answer...