When the engines go into the shop for scheduled inspections or repairs the turnaround time is horrendous for a number of reasons. What a company would normally do is rent an engine and install it to keep the airplane flying but there's no rental engines available either. Back when strictly doing engine changes paid for by the manufacturers engine program was my bread and butter it was very rare to remove an engine and just wait for it to come back. We would almost always install a loaner, that loaner did not normally come from the manufacturer, there are companies out there that own engines and no airplanes. They exist on just renting engines. My experience is all corporate/private/business jets but I'd imagine the same business model is used in the airline world. If a rental company looks at the bottom line and thinks they can profit from renting engines that are as hard to get as hens teeth and they have the capital to do it why not buy fairly new jets from a sinking company and remove the engines and sell the carcass to the highest bidder? As I said before I think long term the engine issues will get worked out but currently if you have a couple of those engines available for rent you can ask a very high premium, you'd be printing money short term and have the asset when the market settles down.
The monthly rental cost from Turbomeca/Saffran for an interim Arriel engine of nearly any model, was indeed charged at a high premium, for the time that the owned engine was being overhauled and damage replaced. That was just base cost. There was additional charges per X amount of hours put on the engine while renting it. Much like mileage being charged when renting a car.
Insofar as the Spirit planes, looking across the runway from our hangar, the northwest scrapping area on the field is mainly scrapping 319s, but there’s a few 321s there too that seem to be getting parted out, along with 320s.