Sad news indeed. If you lived in Europe, you would fail inspection and the car taken off the road permanently, as in scrap. The USA is a bit more lax, to say the least.

My 1995 850 has rust on the frame rails, not visible because of the plastic cover that runs from front to rear. But there is a bunch of rust. This is not repairable and will eventually doom the car to the scrap heap, this being a very salty state.

Your car has just had a heart transplant! And now there is severe cancer diagnosed. Some of the cancer can be cut out and replaced with new parts, but not cheaply. And there is probably more cancer if someone pokes a steel rod into the underside while on a lift (what they used to do in Sweden if rust is suspected).

Whether your beloved 850 lives or not is up to you. You have limited options: Take out the rear seats and JB Weld some metal plates over the holes to keep out the draft and water, as a 2 seater, haul anything car. Have a professional fix the current holes with mega bucks and hope nothing else falls off. Buy an 850 wagon from the southwest that needs a new engine/tranny and swap in yours (not quite the same as keeping your car).

I wish there was better news...
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Keeping it running is better than buying new