Try the retorque first
Yes they are stretch bolts, but they are steel and and extra 20 degrees or so will not cause them to break. Getting them to move in that direction might be difficult, although you mentioned the dealership already tried so they may move easier.
Are you getting oil in the coolant, or just an external leak?
If it's in the coolant, don't discount the fact that it could be a broken trans cooler within your radiator.
'91 300E, 215K miles