Well, I went in and talked to a service adviser at the local MB dealership today and I'm not really sure what to make of it. He was very nice, even though I think he thought I was a moron, I didn't do a very good job of explaining my situation. He said they would be glad to do an alignment, but that he doubted that was the issue and that he would recommend experimenting with the tires before spending money on a second alignment (i.e., try rotating them and see if it does anything different, spin each and check for defects, etc.). I showed him a printout of your 124, gxsr, and asked him about the total toe and steer ahead, but he didn't think that was likely the problem, and that the independent shop probably would have noticed those issues (he did know of the shop and said he thought they were very good, the second-best option for a MB alignment, with the dealer being first). Of course, all he had to go by was my description of what the car does. To be honest I have a hard time imaging the tracking would be that affected by the tires, even though now both my indy and this dealer adviser have said that. Or at least, I'm not confident enough to go out and buy $600 worth of tires to replace brand new tires. The alignment is only $90 if I decide to do that first instead.
He did reiterate what my indy and others have said in that he doubted it was my work that caused the problem ... if the new frame were bent or something installed wrong, it would have been very obvious during the first alignment. He also asked about wheel bearings, front suspension, steering linkage and so forth, which was nice, he took his time to help me think of other things to check first.
So ... for the heck of it I'm going to switch tires front to rear, and I'll check each for inconsistencies or uneven running. Other than that I'll think about it ($90 isn't very much ...). It's very drivable. It's amazingly comfortable and smooth and sometimes I think "wow, I'm nuts, this car tracks perfectly" ... but if it's windy/breezing or I'm on a truck rut ... then the flakiness starts again.

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