What an interesting thread!
I've done the idler arm bushing this summer (last summer) on my 300TE (euro)
without reading anything about it before. The euro M103 also have the additional converters (pre-cats) behind the manifolds so the space for removing the bolt is very limited. I had to take off the exhaust.
Removing the exhaust is not as difficult as it sounds, four bolts to the manifolds and some rubber bushings in the rear - off it comes in one piece. I had the car on the lift so it was quite easy. For the bolts going into the manifolds you need a 13mm socket and a long extension or two. I sucked the nuts and the bolts with some anti rust oil (Rostloeser - don't know the english word now) and got them out without problems.
I don't think it's a good idea to mount the idler arm bolt upside down. In case you leave the nut for whatever reason, the bolt will slip out and steering is impossible. I know, there are many tricks in the workshops to save some time but not all are suitable for safety components like steering and brakes.
You don't need an alignment after this repair. As Bruce B wrote, the geometry will not change.