We have the shifter feel/return-to-center issue finally pinpointed!
Trans #1 had a seized pivot on the shift rod from the start. We just didn’t know it because it was our first experience with a trans from this family. With trans #2 (an MV5 I bought to take comparison measurements from) and trans #3 (an MV7 to replace the one in this car) in our hands it became immediately clear that these pivoted differently than I recalled #1 doing. With the car back in the shop and on the rack I was able to disconnect the shift linkage last night and confirm the difference.
That pivot allows for the off-axis motion the shifter makes as it moves side-to-side. Without the pivot the shifter is binding with just about any motion.