What do you mean by installed backwards? If you mean installed upside down with the sensor end pointing up out of the block that is a problem. Was it upside down or rightside up. Orientation of the thermo when it is installed upright shouldn't really cause what you are experiencing.

Some questions that need to be answered before you continue.
1. Is the oil contaminated with coolant now? If so big problem.
2. Have you properly filled the system, warming the engine with the cap off until the level dropped (thermo opened) then continued to fill up? Already posted.
3. Have you checked for compression gasses in the cooling system as already mentioned on several posts? (rapid pressurization, pressurizing the system so that the cap relief blows, coolant test devices, etc.

It's frustrating trying to help from a distance.

12 Rocket Roadster
03 Bonneville America
69 BSA Firebird Scrambler
73 Yamaha TX 750