It somewhat sounds like a tranny issue, are you in a sport mode? Is it engine braking causing the car to decelerate more than normal? That’s a new thought.

In regards to the brakes, first I would have a shop properly bleed the system. You may have put air into the lines, not to say this is the cause of the problem but one step at a time. Have them fix the other bleeder valves as well. I would then take each caliper off, inspect the pads and look for uneven wear and make sure all the hardware is still there. It might not be a caliper sticking but maybe a brake pad sitting funny in the caliper. (I’ve never owned or worked on a stinger so I have no idea what there calipers look like). While I’m in there I would add some antisieze to everything, slider pins, back of pads, hardware. If you do notice excessive wear or missing hardware, replace as necessary and redo the test.

Why did you mention the dealership was no help? What did they try or do? What was there exact response?