I agree you should think about disconnecting the
oil cooler. I just installed mine and ran braided lines into the engine bay and capped them for future use so it's just there for looks. My
oil has difficulty reaching a high temp as it is, so I'll never need the cooler. The thermostat is just added complexity and potential for leaks.
On the preoiler, it's not a bad idea but I don't think you need it.
On the return line, do you have a mechanical or an electric pump? If electric, you should have a return anyway. If mechanical, I wouldn't add a return. If you have vapor lock issues (esp with winter gas), insulate your fuel lines and add a phenolic spacer. That will likely solve your problem.