ctor
Yes, a Y connector works, and I have used them many times successfully over the years, even though purists make faces like they just smelled dog poop. I never have, or would mess with your impedance idea but you are more advanced than me.
You may be right, because playing the Grado 'dual mono' cartridge output thru both channels thru a pair of speakers does sometimes give a subtle illusion of location, not frequent, but it does occur, not disturbing, just huh? when you notice it, then you forget it, and just listen. Mono well recorded can be excellent and thoroughly involving.
My single mono amp hesitation is playing from only one speaker that is presumably near a corner of the room normally for stereo imaging and located precisely for room frequency balance. Mono Jazz LP's is frequent for me, a few of us in listening positions for imaging, i.e. Oscar Peterson, play this Stereo LP, then that Mono LP, back to Miles Davis Stereo, ... which is my frequent habit, and why I am truly enjoying the two arm large plinth solution.
When they knew Stereo LP was coming, before 1958 (tape went stereo in 1956), the big studios sent two separate teams, their accomplished mono recording team, get a session done, then a separate recording team of new Stereo whiz kids, repeat the whole process, costly for sure.
I read that Rudy Van Gelder observed this, couldn't afford double ..., so he decided to record in Stereo, and mix mono from that, and have the Stereo masters for the future. Clever Indeed.