I would think that the lower control arm bushings are the problem.
If they are shot then when the weight of the car is on the wheels the caster and chamber will be off due to the movement of the lower control arm. With bad LCA bushings the LCA can move inward towards the center of the car as the weight is on the wheels. This will throw the aligment off.
Bearings. Yes they could be bad. But usually you get a grinding or groning when they are going bad. You can also tell by rotating the wheel when on the lift and feeling and listening for smoothness in the bearings.
Have you felt the wheels after a couple of hard breaking applications. If one wheel is hotter that the other then the hotter one is braking more that the other one.
I would suggest getting a second opinion. Your mechanic may be right, but a second opinion will help.
Additionally when was the last time the rubber brake hoses from the caliper to the body fender been replaced. They can go bad inside the hose and trap brake pressure during applications. You can't tell by the outside of the hose. But if they are old this could also be a problem. While driving they do not allow the brake hydraulic pressure to release and as you apply the brakes the next time one wheels pads are closer to the rotor and you get harder braking on that side. When the mechanic puts the car on the lift sufficent time has gone be to allow the bad hose to release the pressure so the wheel both spin correctly and seem fine.
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car