(1) If the chemical test for CO2 in the cooling system is negative, then a head gasket failure is probably not the cause of any overheating problem...
(2) "open fuel pump circuit" means that the computer is not seeing any voltage present on the wire that leads to the fuel pump relay. The fuel pump is grounded directly to the chassis somewhere in the rear of the vehicle. It receives power from the fuel pump relay in the front of the vehicle. The relay primary side is powered up by the ignition switch; the other side of the relay primary coil is grounded by teh computer to trun the relay on. If the computer does not see any power present on the wire from the relay primary, it assumes that the circuit has a problem somewhere (open circuit = break in the circuit somewhere).
(3) If a used computer was installed, it should work OK provided it was the same service number as teh original one and the calibratior was changed from the old one to the new one. It is very likely that the calibrations changed from one model year to the next, so if the correct calibrator containing the software that tells the computer how to run was not installed then the engine may not operate correctly.
Another thing to keep in mind is that GM had common problems with bad solder joints developig on the circuit boards in their engine controllers as they got older around this time, so if it was a used junkyard computer there is a good chance that the replacement one may have problems also.
(4) It is unlikely, although not impossible for a second CTS to fail. It is more likely that the CTS is OK (take a resistance measurement at several temperatures and compare it to published specs for the sensor) and the CTS code is being generated due to some other problem in the circuit, such as a broken or shorted signal wire, damaged connector or pin at the ECM or CTS, or a faulty ECM itself. I am assuming you are using a good quality replacement sensor, not a discount store part or a used one here; that would greatly increase the chance of having a problem with the replacement one.