Thanks apple pie, I just don't have the energy right now to take down yet another lath ceiling. Is there something I could do temporarily that will help me insulate the ceiling without much work?

I was even thinking of using the insulating spray foam product to put along the cracks, but that may look ridiculous :/

You can get estimates from a couple licensed local contractors to put up wallboard, come winter time. It looks like a small job for pros who have the equipment and know-how. If you think the damage might be covered by your homeowner's insurance, check with them first.

You can ask the contractors if there is a temporary fix until they get around to scheduling your job. Don't take chances with electrical wiring safety.

How about something more diy? Any ideas?

I find it hard to believe that the lack of wood is the heat issue. How deep is your fiberglass insulation? Measure it and look up how much is recommended for your climate. If you've got enough, okay, if not, add more fiberglass.

I would guess that the fiberglass is about 10 inches deep...

We did look up there with an infrared camera (during the summer) and you could see the really hot spots through the cracks.

Perhaps you will could get some more DIY suggestions if you post several photos taken with the flash on that show the situation -and the wiring- clearly. The existing photo is very dark - try reducing the contrast by not putting the sunlit window in the center of the camera field of view.