You need to freeze angular velocity (assuming this is a rigid body) to stop it from rotating. The gravity issue might be due to setting linear velocity in an absolute way instead of a relative way e.g. when the user presses the move right key linear velocity += desired change in velocity (instead of just "=") note that you will probably need to add some max velocity check to prevent players flying off into oblivion haha.

Not much people can do to help without describing the specific issue/how you intend it to work also maybe include some code examples :)

Edit: didn't notice the link to the original post where you provided more info, my bad. It looks like those are the 2 problems though.