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Did you do the paper towel test? In the old days mechanics would put a paper towel over the exhaust of a smoking car to catch some of whatever was coming out. If it's steam, it will condense in the paper towel and leave it wet. If it's fuel or oil, it will leave soot.
You said you checked the oil for discoloration, but modern oils have such good detergent packages in them that it's not always easy to tell by looking that you have coolant contamination. In old oils, the moisture would emulsify in the oil and make it look milky. In new oils, the detergents bond with the moisture rather than leaving it in suspension, so that it doesn't necessarily change color dramatically. If you're worried about contamination, the only reliable way to know is to pull an oil sample (hot) and send it off for analysis at a company like Blackstone. That costs $25, and they'll send you the sample collection kit for free.
If coolant is leaking into the combustion chamber (which would be necessary for it to come out the tail pipe), you also may get combustion gases leaking the other direction into the coolant. There is a $50 test kit you can buy to test your cooling system for the presence of exhaust gases. Some mechanics have them and will often do the test for free.
Seems like the first thing you need to verify is what exactly is coming out of your tailpipe. That will help you figure out where to look.
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