Thermistor is a two wire resistive temperature device. If you read resistance with a volt-ohm meter, it is probably good. Open circuit or short circuit is bad. Usually they are 100 Ohm, 1K Ohm or 10K ohm. not sure on this particular one.
The Hayward Turbo cell 3, 5 , 9, and 15 physical dimensions are the same and requires no plumbing mods to interchange.
Correct, the Aquatrol RJ model does not need a flow switch. And yes, you can install the T-cell horizontally but why would you on a RJ? Please take a gander at the Aquatrol User Manual