Regarding your monovalve questions, you might not compromise heat function with the downstream splice, but you will compromise AC function. The monovalve will not close properly with the excessive pressure of the heat return line downstream, which I think is where you are describing your splice. Somewhere on this forum is a thread about replacing the monovalve with a 70s vintage Ford vacuum valve, activated by a vacuum relay (actually an overboost switch or relay) energized by the monovalve circuit. Really its quite simple to retrofit. The problem can also be dealt with by installing a summer/winter shutoff upstream from the monovalve, but the retrofit is a better, no maintenence, solution and works great. You're in Michigan, so AC might not be an issue, but it is in Louisiana! And I concur with checking or just replacing the T-stat. That might solve the problem right there.
Mine is plumbed as described above, t-d out of head, then FPLE, then heated filter, then around back of engine bay t-d downstream from Mono, as seen in this photoset:
http://www.flickr.com/photos/12666884@N00/sets/72157621660693605/