Ball joints, tie rod ends and where the idler arm connects to the center link, all use a ball and socket mechanism to allow for movement when you're driving. When the fasteners are torqued, the joints bind, (normal). Whenever the steering wheel is moved, the steering linkage will tend to return to the position that the joints were torqued in, hence the term "memory steer". If caster is set too low, the vehicle will wander and require constant correction to keep it in a straight line while driving. This is sometimes misdiagnosed as memory steer.
Hopefully this will answer your last question, then get a wheel alignment
The steering wheel being off center is to be expected if you don't have an alignment done after replacing
any steering linkage parts. The steering gear has a center point. There is a high point on the gears that cause them to bind very slightly, (normal). When adjusting toe, the steering gear, (and hopefully the steering wheel), must be centered and locked in position. With S & T body vehicles, the steering shaft is NOT keyed so its possible that someone removed the steering wheel and did not install it correctly. When the wheels are centered and locked in the straight ahead position, toe can be checked and adjusted. Adjusting the toe to specification will center the steering wheel also, if it's done correctly.
When replacing the idler arm, the center link must be level with the frame of the vehicle when the idler arm mounting bolts are torqued to the frame. The holes in the frame are slightly oversize to allow for this adjustment. Those bolts have nothing to do with memory steer, the connection to the center link DOES. If the center link is not level when installed, the vehicle will encounter "bump steer". Bump steer is defined as the vehicle will change direction on a curve, over a bump, without driver input on the steering shaft. In other words, you're going around a curve, holding the steering wheel steady, the vehicle goes over a bump in the road and the vehicle will feel like it's "jumping sideways". Steering linkage and wheel alignment both have to do with geometry, if it's not symetrical, you're gonna have a problem.
This link goes into detail about several problems that can be encountered with steering and suspension:
http://www.aa1car.com/library/steerpul.htm