Congratulations on you persistance.

On another thread with another soul that had a stubborn case I suggested shocking as soon as the sun was off the pool in the afternoon and checking again just a few hours later ... before going to be.

I reasoned that if the pool is heavily loaded with organics there would be a very fast drop in the FC levels very shortly after the shock dose, and that by morning the levels had dropped off enough that things were growing again.

Fortunately, I have never had a visitation from a stubborn algae and could not test the idea, but your case and the method you finally got to, lend some support to this idea. I am thinking that we might need to fine tune the suggested method used to fight an algae bloom to short cycle the retesting and raising the FC levels compared to the proceedure that has been generally suggested.

1) find the break point needed after testing CYA etc and use bleach calc to get the right does.
2) test withing a few minutes of adding to make sure you got it right.
3) test again withing a couple hours to see how fast it is dropping and repeat the process. The faster the retest and re-load to shock levels the faster the algae will be killed.
4) keep it up until the FC levels show little change in retesting after sunset to take sun breakdown out of the equation.

Your experience may have been a turning point in the methods we use.