If it were me, I would attempt to find another union that matches up with the one coming right off the return from the filter. Then use hard PVC to come straight off that union (glue the correct end on that piece to screw onto the other half of the existing union....or replace the line coming off the filter and use a new union) past the filter tank, then vertical section of hard PVC *with the SWG cell in that section*, then elbows over and down to the existing inlet into the 2 return pipes.
I *think* the cell can be installed vertically (IntelliChlor, right?) I know they show a bypass loop setup in their manual, with the cell installed vertically so it must be allowed. They do recommend 12-18" of straight pipe before the cell to provide for "best flow sensing". I think you have enough room in there to do that.
