I still have my copy of Voyetra's Sequencer Plus Mk. III, which as you say runs under DOS 2.0 and higher. SP3 requires a hardware key which plugs into the parallel port. Back then, I also ran Dr. T's Copyist program for notation.
There was another program called Band in a Box that I remember seeing here and there as well? I always thought it was an auto-accompiament program but never looked into it farther. I think it got packed in with some of the Soundblaster clones. I remember my high school band director hitting me up for underground copies of scoring programs to play with (but I don't think I had any nor remember playing with any, this was the early Windows / MS-DOS days.) I started to mess with trackers and that is where I mostly stayed. ScreamTracker (From FutureCrew), And Composer 669 (which was from the demo group Renaissance which I believe was from New Jersey!) Interesting thing is that a lot of software can play the 669 tracker files but there is something that someone got wrong in the numbers somewhere so some of the portamento effects are just... wrong... on everything that plays the files other than the original software. I suppose everything is a derivitive work of something that wasn't 100% right. There is a really powerful modern tracker called Renoise. I keep meaning to buy a copy. I wonder if OP found Windows 3.1 sequencing software? This whole thread had me looking for something to put on my Mac SE/30 :-)