If you have a tap, run it in to clean the threads. If you don't have a tap, make you own by cutting a groves down the threads from head to end of the bolt. Grinder, file, hacksaw.
Yes fresh air from the airbox. I've tapped into the 1" dia hose going to the IAC and also directly into the airbox on cars that do not have the large IAC hose.
Yes KYB are good struts. Another person posted a like issue a month ago.
Compare the old to new. Are they the same with no weight?
If so the spring rate may be different.
If you step on the front bumper, do you get normal compression?
Sounds like you still have air trapped in the system. Temp switch for the fans is up high on the crossover pipe on top of the engine.
Is the temp gauge operating normally? Good Subaru, warms quickly to the same temp every time and stays there.
They all should be the same housing. Did you compare the old to new?
I've replaced many and as long as the fan is clocked correctly all holes will line up.