I tried all three methods: trim plug-in, using a send and tapping one side of a stereo bus, and they all load fine when I restart DP. I'm not sure why DP is behaving this way for you.i opened mx4 stereo instrument track and bussed it to a mono aux track... works great until i load DP back up... DP won't find the MX4 instance and asks to clear it...
Of the three methods, the trim plug-in is by far the cleanest, because the channel is self-contained. I prefer to mute one channel by turning the gain all the way down, and center pan the other channel. This method prevents any odd summing to mono problems that may occur, but may not work well for patches with pan modulation. The end result is the same as if you took a mono signal and bussed it to a stereo output.
With DP 4.52, sends also provide the option of summing to mono, or tapping a single channel.