Does your van have the factory installed oil cooler? That routes oil from under the oil filter adapter to a cooler in the rad.
I am just trying to think where oil can get into a pressurized cooling system-the oil has to be under higher pressure for that to happen. So far the only place I can think of keeps leading back to the rad, but hopefully others will make suggestions.
Using brake clean on the parts rather than acetone should be fine. The main thing there is to make sure everything is meticulously clean and free of any oily residue before reassembly and using the RTV
Jeff