Yes, I remember your car. It's managed to survive this far, so it's not doing too bad..
There's a few problems by the sound of it, which makes it tricky to work out whether some of the symptoms are caused by one of the problems, or if there's a separate problem for each symptom.
From what you described;
1) It does sound like there is a "cylinder" problem. If you do a compression test you should get a series of readings that are consistent across all cylinders. If you have a cylinder or two that have a lower reading than the others then they have a problem. If you carry out a wet test on these cylinders and you get >10% increase in compression, then that cylinder has bore or piston/ring problems. If the wet test gives <10% variation, then you have a valve or valve seat problem.
Worn seals don't affect the compression ratio, so if your engine checks out on compression but you're burning oil, then you could still have worn seals. The easiest way to check for leaking seals is to take the cover off and visually inspect the seals... or take the plugs out and see which one is oily... or put an endoscope into the spark plug holes to see which ones are the most sooty.
If you have two adjacent cylinders with low compression, that are both oily, then you could have a head gasket problem, leaking oil into one or both of those cylinders.
I remember (I think) that you carried out a compression test. Do you have the values for each cylinder?
2a) If you have oil being drawn into the cylinder then that causes problems, and not just for Greta Thunberg. If the car is constantly sucking oil, then it'll try to burn it. The affected cylinder(s) will run rich so the engine will "race" or idle funny or hesitate under light loads. It'll run well initially, when you boot it, since a transition/accelerating engine wants to be rich, but it won't rev out because it's too rich. But it's too rich with a fuel (the oil) that burns slow and "cold". It's actually lean with the petrol because the lambda is detecting that it's rich and will be telling the ECU to back off the mixture.
Oil burn also eventually knackers your lambda sensors, since they don't really like oily soot. A knackered lambda is not going to solve the fuelling issues better than a working lambda.
How much oil is your car using, every 1000kms for example.?
2b) If you just have a valve stem "seep" then that oil might make the car awkward to start and idle, until it's burnt off but then it should behave reasonably normally, depending on how bad the valve seal leak is.
3) Valve seal restorer *might* work if the leak is a small one caused just by hardened seals. If the seals are physically damaged (pieces missing) or the oil burn is something else, then the answer is "no".
As a simple test, you could put some restorer in there (cheap and simple test) to see what happens. If it reduces the oil burn then that might sort out some of the other issues.. in which case, even if the other problems come back again later, you'll know that the oil burn is causing the problems and a new set of valve seals will sort it for good. If the seal restorer stops the smoke but that doesn't solve the other problems.. then the other problems are something else (although if the oil is killing the lambda sensor etc. it's still the chief culprit).
4) Clutch slip.. agreed that it sounds like clutch slip. I would sort out the main problem which is potentially a failed head gasket and/or moderate valve/seal problem first, since if you know what that is, then you can work out how to fix it. If you decide that the engine is too knackered to fix it (e.g you decide to put a transplant in there) then you can fit the new clutch when you do that, not before.
5) The other sensor errors *could* be a result of the commotion inside the ECU, trying to work out what's going on and coming up with "out of range" errors/readings... so these might be solved if you discover you have a serious oil burn problem and fix it. Some of the sensors could obviously be affected by the engine's problems (soot and oil where there shouldn't be any).
Ralf S.