yes, the centering method I gave you earlier was for dual pivot calipers, which it turns out you don't have. I haven't seen the problem as often on V-type brakes. These would require a decent amount of imbalance between the springs in either arm to deflect the rim. They can also be pushed to either side by too short or long a housing loop coming from the frame, which is then overcompensated for in the spring settings.

It'll take a bit of detective work to get these to be neutral on the rim when closed but it can be done.

As to the toe in question. It will move when you loosen the shoes, but rear brakes require little or no toe-in anyway, because the rear pointing bosses don't flex to a toe out position under load as much as fronts do. If you want toe-in wrap a rubber band around the back of each shoe to hold the heel out as you position and tighten the shoe. You want no more than the thickness of a dime.