Castor is typically desired for steering effort and helping with the self-centering of the steering wheel, but has little effect on tracking. The self-centering is when you turn the wheel & instead of having to straighten the steering wheel, angle of the strut & forward motion of the car "unwinds" the steering wheel itself and the car goes straight.
Wandering seems like a toe issue to me. If you guys were shooting for near zero toe or a bit of toe out, the car loses some straight line stability.
If your toe checks out, you may have what's called a "radial pull" induced by a tire? Sometimes the finished alignment specs-out good but will leave you with a little bit of pull.
Instead of using cross camber to correct the pull, its better off left alone. I believe the reason being is because the cross camber can affect tire wear and shops don't want you coming back saying that a bad alignment job was done, look at my tire wear? So they choose the lesser of two evils- a bit of a pull vs. tire wear they have to defend. In your case since you know the guy, maybe he'll give your money back if you just go away?
Yeah its a good idea to find something to replace the tar goop to keep water out of the trunk. Rope caulk (like non-hardening clay) used for window sealing works but its typically white & looks sorta crappy on non-white cars.