Hello I'm Neal and I am here to help resolve your issue. If I cannot help I will opt out and refer your question to another expert.
Start with the temp sensor for the computer, it is likely bad and that is causing the fan to not come on. Let me know what you find and we can go from there.
What are the chances this ruined the radiator, also used antifreeze that may have been very old after this occurred, though Prestone says it should be good for many years, should I flush it... Not employed and have limited ability repairing cars.
The fluid used was sitting in storage for 5+ years, it had never been used and it wasn't premixed with water.
Do I have to drain fluid to remove the sensor or can I just remove it and wait till till it stops draining before doing the replacement?
The radiator was cracked (plastic top where it joins the aluminum body) and replaced as well as the hoses and thermostat and fluids.