What might be wrong with my sewage ejector pump?
If it has been working fine and this is a new problem then it seems like your waste line is plugged. Basically what's happening is your pump is pumping the waste and because it will not go through the waste line it's going up the water softener drain. The fact that water drains into the pit really isn't relevant. Also the upstairs toilet may tap into the waste line after the blockage, which is why that toilet works fine. If the vent is plugged it may cause some draining issues but no draining issues if the waste being pumped out. From the picture it looks like you have a good spot to put in a clean out so you can rod the waste line.