Seems to me that the four bolts....one on each side of each valve....come out and the whole shebang is in your hand in about 2 minutes.
My guess is that your problem with idling is going to end up being carb "related". But you make an excellent point about operating temperature.
I'll keep that in mind on the carburetors and I'll remove the extra few ounces of oil.
So is 160°F normal or is the thermostat malfunctioning?
I have a 1996 Mercury 150 HP with about 100 hours on it. A lot of my problems that I have had are lack of use. Started to go & went maybe 20 yards