Just a couple of other thoughts:

First, have you pulled and cleared all the codes? There could be a code that points to the problem. Intermittent neutral safety switch failures have been known to occur.

Second, ruling out the starter itself being bad, some of the starters on these cars have shims on them. Whether or not the car has shims is dependent on the individual car. Some have none; some have one or two. I don't know if incorrect shimming would cause the solenoid to stick, but it might be something to research.

UPDATE: Also, although the fuses may look good, looks are deceiving. Pull out fuse #8 and check it carefully. If you don't have a replacement fuse, clean up the ends with some fine sandpaper and reinsert.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles