Overheating and questions thermal sensors

You are correct as to coolant flow direction. I think you will find that replacing the radiator will make the problem go away for another 25 years and with a good radiator, the factory gauge is all you need. The inside of the radiator tubes just get corroded over the decades and no chemical will really do a very good job (well, maybe Red Devil lye but that might just eat clear through a spot that's already been corroded thin). In the 'good old days', radiator shops would unsolder the top and bottom tank and use pieces of brazing rod to 'rod out' each tube. The price of radiators has not gone up as fast as the cost of labor to fix old ones so a swap is the best choice. I'm sure that aluminum thing is an oil cooler as you suspect. A new radiator is what you need.