Wait for the correct parts before putting it back together. Brakes are designed to be as idiot proof as possible; if they just started making noise, you've got time. Besides, brakes aren't hard to do in a driveway

The rear main seal housing gasket is designed to go on dry. You should be able to remove any and all of the old gasket. You're taking gasket material off of a cast iron block; find something harder than the gasket but softer than iron. I find the Permatex gasket remover spray to work excellently along with scotch brite or a utility knife blade. If you're still worried, a VERY thin coating of RTV won't hurt.

Don't shim your new pivot ball.

Looks like some great progress, and good work with the airbag cover