The most likely causes for the S10 Blazer to overheat at highway speeds but not in the city are a failing radiator fan clutch, a clogged radiator, or a failing water pump. At highway speeds, the engine is working harder and generating more heat, which could overwhelm a system that is not functioning properly. It's recommended to have the cooling system checked by a mechanic to diagnose and fix the issue.
Your cooling fan is not engaging turning on perhaps checking the wires to the fan and making sure power is getting to it. May be the fan clutch or whole fan itself, possibly a thermostat or water pump check all of it
Once you have achieved highway speed, you would stay in the highest gear, except for steep uphill or downhill grades.
If it has a thermo(electric) radiater fan, check that it is working, by connecting it directly to the battery, and if it works, it's probably not switching on when the temperature rises, so the thermo switch will be faulty.
It depends on how hard the pull is. It would be safest to get your vehicle fixed before getting on the highway.
I would get about 34 mpg driving mostly highway
driving on the highway it would be about 10 to 11 hours
The total current in the circuit would be reduced by the amount of current that was supplying the bulb. No other bulbs in the circuit would be affected. The comparison is driving on a highway. If you are driving on a two lane highway and the car in front of you slows down, you must slow down. If you are driving on a four lane highway you may drive beside (parallel) to the car, so if that car slowed down you would not be affected.
No, not an alternator itself. A slipping belt driving the alternator and the water pump could though.
Driving gently you might be able to get up to 15 or 16 on the highway, around town would be less.
It would shorten the life span of the engine some.
You are not giving enough information to get a reasonable answer. It's like asking why it won't start. Any, and I mean any, detail would help. Are we talking about the engine cooling system? What are the symptoms that bring you to the conclusion it is overheating? Have you checked the coolant level? What year and size engine? Does it overheat only when stressing the engine, like climbing hills? Does it overheat only in traffic and okay on the highway? Does it overheat when the car is idling? Has the head gasket ever been replaced?