Using a mono 1/4" plug into a stereo jack still is shorting the two channels together. I actually blew the left channel out of an old sound card plugging a mono headset into it. But other headphone circuits are more forgiving. So it could work (the Y on the RCA could also work), but electrically it's still not the right thing to do.
Most music has the low bass mixed in mono, so driving your sub from just the left RCA will still get you most of the bass information. Then you can get a second sub and run it from the right.
(Or the summing box.)
