No don't think fossil fuel contamination. Think heat contamination. On a cold day you can see your breath. Cars that are just started often have white smoke coming from their exhaust pipes. It's all the same thing. Air at high altitude is cold. Jet engines have a hot exhaust. It's the hot exhaust mixing with the cold air that causes the water vapor in the air to condense and produces a cloud formation behind the aircraft. There isn't much soot from the exhaust of a jet engine, most of it is just hot air. You can also see vapor formed on the top of the wings of a fighter jet if the jet is pulling a lot of Gs in a maneuver. There is a low pressure area on top of the wing and it's below the vapor pressure of the air so you see a bit of a cloud for an instant.
Just a point of clarification, hot air being introduced to a cold environment doesn't result in cloud formation. Hot, moist air equalizing with a colder environment produces clouds. It's the warm, moist air in your breath cooling down to it's dewpoint that produces a cloud. The humidity of the environment doesn't have much to do with it. If I wave a blowtorch around on a cold day it won't cause any clouds even if the temp/dewpoint spread is less than a degree. I think that was what you're saying but I wanted to clarify. I probably wouldn't call it heat contamination either since it depends on the characteristics of the hot air mass. Now a jet engine is kind of like your breath example. The combustion process does produce some water, but it's not enough to form all the clouds you see in a contrail.
With regards to wingtip clouds and the like, that's a different phenomenon. If a jet engine produced the same kind of conditions as a wing at high angle of attack then you should be able to see contrails more consistently at high altitudes.
And with soot, we are working to producing less of with different alternative fuels but I've seen firsthand what happens if you don't wash an aircraft for a week or two. The tail's black as a chalkboard.
Also, this is an older study but it's a good read regarding the effects of aircraft soot on cloud formation.
https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001GL014115