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that's a hard question to answer actually. here's why:

that film is actually very normal. condensation will naturally settle inside the engine and once it heats up it rises to the top - where the fill cap is. you see it more in the colder weather due to the bigger temperature swing.

you are correct in saying that the 'chocolate milkshake' under the oil cap might indicate a blown head gasket. coolant enters cylinder and gets burned with the fuel/air mixture. not all gets burned. some will eventually get into the crankcase. it's usually more than a film...

my thoughts are this: normal thin condensation film. your coolant level dropping after changing it can mean there was air in the system that worked its way out. a blown head gasket and you'll notice a continual drop in coolant and that film will get worse.

as far as the running rough, that should be addressed separately. if you're really concerned about a BHG, do a compression test. if you can replace spark plugs you can do this test and the tool is not expensive.
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