You wont damage the bearings by wiggling it. If it has enough compression, you can just rotate it the way it would turn if the engine is running and the compression should hold it for you to loosen it. Did you drill the prop out before you put it on? The fit should be snug but not that tight.

On the inverted engine... its probably not fully broken in yet.

An engine can be fine upright, and a dog inverted when its still breaking in, and that can take longer than many people expect.

Some .40 class engines still aren't fully broken in after 3 to 4 gallons of operation. (That's a lot of flying...) Most will stabilize sufficiently for short inverted ops after 1/2 gallon... but for inverted mounting, I find it better to use an engine with a year or two of operation history.

If all else fails, remove the muffler and put a piece of hardwood (broken prop tip maybe) thru the exhaust port to jam the piston while you turn the prop off. The wood will not damage the cylinder or wall.

I have removed the plug and fed nylon cord into the cyl to allow the prop to be spun off. Once off, reverse and cord should slip out. Never had any trouble with this method. Best thing tho would be to have a prop reamer of the proper size.

Jimmy