One thing that I don't understand is how come UA/UA Express/Agents apparently didn't know that there was a requirement to get four crew on board, until after the passengers had boarded. Surely if they had known about it, they could have boarded (plane capacity - 4) to leave space for additional 4 crew. If they didn't know about it, why not?
Fact of life. Equipment break down. People get sick. Other crews get stranded due to weather. Etc., etc., not all knowable until last minute.