The people who owned WUCO at the time the 105.7 allocation was granted to Marysville made a serious mistake by not applying for it. Of course, there is no guarantee that it wouldn't have eventually migrated into Columbus even if they had, but at least it might have had a better chance of remaining in Union County and actually serving it.
As for the possibility of 104.3 being changed back to a local operation serving Union County, it would be pretty slim because most potential buyers would be looking at the $$$ of trying to serve at least part of the Columbus market. The only chance I see of it becoming a local service for Union County again would be if Saga bought one of the new move-ins (101.7 or 102.5) and sold 104.3 to a local owner in Marysville.
The only reason Saga has held on to WQEL was to move it to Marion, which they are currently working on.
Going back to the question of any other rural FMs going mono to squeeze their signal farther into a metro market, I can't think of any, but there is an interesting bit of related trivia I can add to this particular situation. When Hot 105 (WWHT) operated in the early 90s from the tower that 104.3 is currently using, the frequency response was limited to either 12,000 or 13,000 Hz as I remember. The reason for that was that Delco and some other car radios then were set up to limit the high frequency response to one of those figures on weaker FM signals in order to avoid a noisy signal. Since that limiting effect could be heard as the signal became stronger or weaker, Hot 105 attempted to alleviate it by just limiting the high end so most listeners would not hear the shift when the high frequency response limiting kicked in and out. It's not as drastic as just going mono, but it was a similar move to deal with the weaker and more distant signal.