Ok, I answered these in order:

Humidity should be somewhere in the 50-60% and can be upped to 70% during a shed.

Some snakes perfer mice, others like rats. Whichever it takes you should keep feeding.

You can have it popped or probed by a vet or breeder. DO NOT try this without experience or you may hurt the snake!

He's probably stressed and maybe hungry.

Sure, fake plants are fine.

Welcome to BP.net!