"The brick steers 'twitchy' at highway speeds...like riding on top a narrow ridge, the car wants to be anywhere but going straight..."

Classic description of toe-out. Sometimes it's described as "squirrely."

It'll chew up the front tires.

"...the front end alignment now, will replacing the rear bushings throw things out of alignment enough to have to repeat the alignment?"

It should not affect front end alignment.

"How do I check at home?"

It should be checked on an alignment machine.

Having said that, I'll also disclose that I built an alignment gauge that I used on my '86 245 (which also had severe toe-out after tie rods and end replacements). When I completed my home-brew alignment, the car tracked dead true. Later I had the alignment checked -- dead nuts on (I set zero toe-in).

"And what may be the corrective action??"

A straightforward adjustment of the outer tie rod ends to align the front wheels -- and then another adjustment to center the steering wheel. All-in-all, for me, it took under 2 hours.
--
Don Foster (near Cape Cod, MA)