I guess I don't understand why the mechanic doesn't step up and fix this correctly... shrug...
Ok, no cost or low cost things to do that will point you in the right direction. Borrow a rear wheel and tire for a test ride. Should take more then 30 minutes to swap and test ride the bike then return the wheel. Steering stem can be eliminated by just re-adjusting the stem torque according to the manual. If you want to go crazy, then pull the top off, clean and re-pack the top bearing. Drop the forks down enough to clean and pack the lower bearing. Re-assemble, torque by the book, then test ride. This next one is a bit more complex, but for a short test ride shouldn't hurt anything. Swap front wheels with another known good one. Yes I know the rotors and pads will be mis-matched but for eliminating a severe wobble, this test ride would also be very quick.
I run Hybrid Tires....
Front: Bridgestone Exedra G709 130/70R-18
Rear: Michelin Primacy Alpin PA3 ZP 195/55R-16