I took my valve covers off on my 2007 Tacoma V6, while digging deeper to look into a possible head gasket issue, and noticed that one of them had a nut missing that went on an upright flange threaded stud (sort of like a flange integrated screw) and was therefore most likely leaking slightly. The cover has about 10 bolts and two of those flange studs (I can't figure out why not use all bolts, maybe to keep the cover still in place so the gasket doesn't shift when servicing).
I have recently had some engine overheating but I know my cooling system works since I serviced it (I know that coolant is flowing fine, water pump and thermostat are working etc). I was wondering if the valve cover leak could have somehow, even indirectly, contributed to the overheating. My intake manifold valves are coated in carbon blowby, which is not directly connected to the cylinder heads but I'm not sure if it could be a side effect. I know that stuff is coming from the PCV and will use Seafoam to clean it.
So, my question is: what are possible consequences of valve covers leaking and could it have somehow caused engine to overheat and break head gaskets. It might also be worthwhile to mention that the truck never overheats in (sub)urban driving at lower speeds, only after about 30 min on the highway driving 70 mph.