If I have an outdated boot image on my standalone media, what will happen? I want to update the boot image with a few things on the primary server but if I do will the standalone still work? I thought standalone media doesn't at all communicate with the site server?

The standalone media does not communicate with the sccm servers at that point so it could not recognize that a cert has been revoked. Your standalone media WILL still function as designed. I wish there WAS a way to kill versions of standalone media instead of pestering my techs to make a new key every time something is updated.