I see two or three points of failure here,
1 - you used a coolant that is known to be bad, it actually will rust out its own container.
2 - you left the acid in too long (if I read that correct - 3 hours)
the hot acid solution is poured in - engine started and ran for 15 minutes at 2000 rpm, drained while hot and then flushed out with hot water at the same time, if you do not flush it out even after draining - citric acid solution is a bit sticky and will keep working. That is why flushing immediately with hot water is very important.
3 - your engine had unknown rust inside that already had compromised the integrity of the head gasket.