I understand your embarrassment and pain
I should have also mentioned another way to check overall flow that would have possibly picked this clogging up. I'll mention now for the sake of completeness.
Note: This test will cover thermostat and flow and is worth practicing on a working system just so you know what the effect/feel is.
1) Start car from cold
2) Let it idle
3) grab top radiator hose in one hand
4) grab bottom radiator hose in the other
5) hang on to both and wait, wait, all very boring
6) top radiator hose will begin to get hot, then hotter and hotter still as time passes
7) bottom radiator hose basically remains cold, cool-ish, luke warm
8) the differential of temperatures between (6) and (7) will be vast with the top hose possibly too hot to hold
9) at some point the thermostat will open and SUDDENLY AND QUICKLY the bottom radiator hose will rapidly become very very hot
10) (9) is almost a binary event.
The almost binary event shows that (a) the thermostat is opening and not stuck open and (b) that there is good water flow through the radiator matrix.
Just by their nature of design most thermostat failures are in the open or partly open state. Were this the case then the bottom and top radiator hoses would increase in temperature together with the bottom slightly lagging the top. If the thermostat is OK and the radiator somewhat blocked then when the thermostat opens then the bottom hose temperature will climb more slowly.
Running this test is a bit like watching paint dry and one is bent over the bonnet area hanging on for 10 to 20 minutes but it is critical that you do so because the binary event happens very quickly and if you miss it you have to start all over again the following day or many hours later.
