If you use a moderate amount of lube, and hand tighten, then torque another 1/8 turn (with towel or tool), and you get a leak, then you're not doing it wrong. Especially if you've tried multiple o-rings.
The reason I suggested the o-ring swop is just a matter of thorough trouble-shooting. If you have an o-ring that you know to be working (the other one), that you installed, and then swap that into the leaking side, using the same technique, and still get a leak, then it is not the o-ring and not your technique.
That would point to a crack you haven't found (or can't see), or a deformation in one or both sealing surfaces.
Are you sure you have the correct size o-ring?
When you hold the two surfaces together, without the o-ring, do you see any obvious gaps?
I was going to suggest this earlier, but then erased it. It is totally radical and untested and I'm not sure it wouldn't ruin your flow switch, or your SWG. Use at your own risk, after you've ruled out everything else! You could unplug the SWG and then flip it around, 180°. If the leak swapped, then it's a deformation in the SWG. If the leak stays on the same side, then there's something wrong with the union.
But, again, I cannot confirm or deny this would work, or ruin your SWG or at least it's flow switch. If it works, I'm a genius, if not, we never met...
@ajw22,
@Newdude, is that too crazy?