I've got absolutely no experience of this engine, but I once tried to buy my wife a used Audi A1 that had this engine in it, on the face of it, that engine seems to have been a jewel of an engine and won lots of praise when it was first launched and maybe even won international awards - but it quickly turned out to be a properly flawed jewel. I think that VW Group did buy back cars that had issues under warranty, and then passed them out to auction houses, from where they were bought by adoring car nuts without any bad history attached, and that cost too many buyers a lot of money. So that was not a very honourable thing for VW Group to do was it!
Something in my brain made me give buying a used Audi A1 with that engine a body swerve, and so I bought my wife a new VW Polo 1.2TSI 110PS SEL, at that time I knew nothing about the possible bad issues with the twincharger, so I think that I dodged a bullet there!
I would have hoped that some VW Group motoring forums covering that engine or its almost as bad quick replacement model, would have amassed enough facts in the form of a "what to do" suggestions, maybe there are buried in the
resource I pointed you to, hopefully, good luck!
Maybe you need to get to know a proper local VW Group Indie - they do exist, and they could try to
guide you in the best direction, or maybe even rebuild that engine to a better standard.
You mentioned spark plugs, yes, the oil being taken into the combustion chambers with mess up the spark plug operation - but while these plugs should get replaced by the correct ones at the correct interval, the oil will do the same to the next set.
One "positive" thing, this was not the only engine that VW Group messed up on, the 2.0ltr and same age 1.8ltr engines, mainly it seems when fitted fore<>aft in cars, ie Audi A4, did end up needing a lot of rework, which VW Group did cover at the time and might still be covering, as long as the cars were serviced within the VW Group service loop - for these CAVE and its "sister", I don't think there is any way to get them sorted out at VW Group's expense even although it is design issues that have caused this.
Finally, I'm guessing that if/when you get this engine sorted out, it will be back to its brilliant best for a while, and you will love it again.