There is a small paper/cardboard gasket that seals the housing to the backing plate. I've actually made a new one out of a cereal box before (make the inner and outer diameter circle and draw them onto the box and cut it out).
The ability to scan for codes tests every sensor & the wiring for the entire system. Even if there are no codes, that at least tells you it's a mechanical issue, not something related to the ECU/harness/sensors. Without that info, nothing an be eliminated. So ideally you either find the diagnostic port or add one. I'd have to look it up but it's likely you can ground a pin or two at the ECU to output the codes through the check engine light without adding the diagnostic port wiring.
What harness is it? I've never heard of an aftermarket 2.3 harness for a stock ECU that didn't have a diagnostic port.
How difficult a new harness is depends on wire routing for your application, whether the stock harness length was retained or not, how comfortable you are with basic wiring (connecting power wires, ground wires, gauge signals, etc.). The rest is just plugging in connectors.
The PiMPx doesn't use the diagnostic port so in a roundabout way that "fixes" that problem.
You'll need to test the three TPS wires to verify you've got ground, 5v, and variable (close to 1v closed, close to 5v wide open). It was likely not plugged in because the previous owner had the same issue and decided the easiest "fix" was the unplug it.