Another suggestion into the mix.......and I'm sorry that it is not going to be good news.
What about a cracked block or head between the oilways and coolant channels running inside the block head which has occurred as a result of a blown headgasket causing the excessive temperature you describe and then the temperature causing the block or head to crack

The explanation of oil in coolant
When the engine starts the oil system reaches pressure almost immediately and will overcome the reduced pressure of the cooling system at cold. This will allow oil into the coolant as the two systems try to equalise their pressures.
I think whatever the theory, you almost certainly have a serious engine issue headgasket failure, cracked head / block and or failed oil cooler. The symptons you describe regards temperature (not oil in coolant) are exactly what I had a work with an Astra, and the diagnosis was ..........blown headgasket.
Check the compression of each cylinder but also get a sniff test done on the coolant to determine if hydrocarbons (combustion gases) are present in it. If they are then it is a very good indicator that the head gasket has failed. The test is easy to carry out with a dramatic almost instant colour change to indicate failed headgasket. See hear for details
http://www.justoffbase.co.uk/Combustion-Leak-Detector-Sealey-VS0061If you do find it is the head gasket, get the heads skimmed or replacement true (level) heads before refitting to remove any distortion, as you dont want to do this job again in the near future. It goes without saying as well do both head gaskets even if only one has failed, and whilst the engine is stripped down this far consider a cam belt kit and new water pump, to save you time in the long run.
Sorry it is not positive news but good luck.
HTH