That's what happens when you create a dependency on ONE person to manage your system. It's insane.
There are no good answers short of a) hiring the guy back, or b) hiring a professional. Not w/ so little time. You can't learn something that complex in a few days. Heck, it's taken me DECADES! I learn something new every day that I didn’t know the day before. It’s amazing, there’s no end to what you don’t know in this field.