Ken, You are right. If there isn't noticable play from the ends that is not the problem. Toe out can cause the cars to follow the crown but it should be similar both directions.
I presume your problem still is with alignment. I hate to say it, but many alignment techs just align to the machines window of acceptance. Few will deel with camber or caster on a strange car that they have little experience with.
And most important, most alignments are done without a diagnostic road test. This is often due to the fact that tire stores are often in the most congested areas of town. I always do a 2-3 mile road test where I can get up to highway speeds to evaluate the ride before I measure.
The nice thing about MB suspension is that except for real poor tires the car can be made to do about anything. Within the specs the car can usually be made to go left or right.
Tires make a tremendous difference and if the car has been in an accident it will drift to the side with shorter wheel base. If aligned with an 8 head machine track width and wheel base differences can be measured.
Bosch Master, ASE Master, L1
33 years MB technician