SWG before heater?

I am putting in a SWG (Aqua Rite T-cell-15). It would be so much easier to put it and the flow switch between the DE filter and heater. What are the disadvantages of such setup compared to the right way of putting it after the heater as the last component in the return circuit? The heater is about 10 years old Purex Triton MiniMax 350.

Other option would be to add valves and bypass the heater when the SWG is on. I could leave the heater to the SPA circuit and SWG in the pool circuit. Would it harm the heater if I let it dry and circulate water only those times when SPA is on?

The high chlorine levels in the SWG output will cause premature wear/corrosion in your heater; I wouldn't even think about it. If you can somehow bypass the heater while the SWG is operating, that would probably be fine.

Recommended location is after the heater and in the pool return line. As Kurt said, don't even think about before the heater.