Wow, there is a lot to unpack there and I feel your pain with all the repairs. To be honest I don't think I have ever really read too much about those types of problems on the various forums. It kinda makes me wonder if there was some substandard suspension work or a lot of water involved...
The short answer is no, the dealer has no way of knowing your engine was tuned if you take it out before having your vehicle serviced. The search feature is your friend!
Here are many hours of reading for you to regard before making a decision!
+1 on the edge CTS2. It allows you to see any codes (I think. I have never had one:)) and also shows your soot % buildup and allows you to run a mobile regeneration at your convenien.It also has a couple other gauge options that are interesting to watch. If you don't want the suction cup mount...
I have seen a lot of threads about problems with the ecoD but I am not sure oil leaks are a major problem...That being said I would always be leary of a used vehicle that leaks oil.