Problem solved! Thanks all for helping me stay determined to find a solution. As I was throwing away the garbage tonight, I noticed I had 3 smaller saddles the kit came with. I took one more look at my equipment pad and found a lonely pipe that just happened to be the same size for the saddles. This is for a RETURN jet on a sun shelf which is rarely used, so there's never any pressure on that line.
Drilled a new hole, connected it to the line, and the water was flowing very generously so I closed the ball valve almost all the way. I tested it with the bubbler running at many different RPMs with dye in the line and it's flowing perfectly.
Red: supply line for the flow cell
Purple: old location of return line from the flow cell
Blue: new return line for the flow cell, connected as far possible down the line.

Just have to go to Home Depot tomorrow to get a 1/2" cap to screw on the old saddle so I can remove the ball valve there that's no longer needed.
Now if only I could find a flow switch that could thread into that old saddle. That'll be my next project.