Engine burning oil (1qt / 200 miles)

I have a 2009 SL with about 78,000 miles on it. Around 60K miles the car started having a audible "knock or slap", especially during the then cold winter start ups. I did some googling and found alot of people saying the pistons shrunk/expanded which the cold would amplify. Sounded plausible and the car ran fine and after it warmed up it was ok.

Around 70K I noticed it started loosing oil alot sooner then I would expect and the knock or slap starting getting alot louder during startup and continued to be heard once warm (though much quiter). At this point the oil consumption started to get alot worse and 3 times I got the oil light.

Around 75K miles I started regularly checking the oil and water level to see what was going on and thats when I was able to measure the consumption. I am burning about 1qt every 200 miles at this point. I took a sharpie and drew 5 lines on the radiator overflow tank and it hasn't moved in a 1000 miles. So I don't think its a headgasket, which sounded like a common issue.

The car has a noticable lope to it at idle and if you brake moderatly quick the car will die when it comes to a stop. Having the AC on increases the likely hood.

I replaced the spark plugs at around 75K miles just to see what would happen. The car did run much better for about 1000 miles. But eventually the same old symptoms came back. All the plugs were pretty dark. So I think the new plugs probably ran great until they got coated with crap.

Soon I will take off the intake manifold and do a compression check and leak down test to see if its one of the cylinders or what.

I'm thinking its probably a cylinder or valve seal issue with how much oil its burning. But since the oil light came on several times and it makes terrible sounds by the time that happens (dipstick is dry by the time the light comes on) I'm thinking I'm just gonna buy a used engine and just do a simple swap. Don't want to run the risk of some of the bearings or seals going bad later. There is no oil leaking on the engine. And there have been a couple times a big smoke cloud came out of the tail pipe when the engine was cold. But once its warm the smoke goes away.

I do have a question. Will the MR20 work with the MR18 ECU and the CVT transmission? From looking at the threads it looks like most of the MR20 swaps were on the manual transmissions. But if its gonna be a hassle then I'll just stick with the MR18 especially since it is a couple hundred dollars cheaper.

I know I'm not really asking for help here, but I was just more interested in getting my story out there and seeing if anyone had similar issues.