Unless I missed it, there has been precious little description of what was going on and how the prison is set up. But I gather there were just two guards, which should be plenty if all the prisoners are locked in cells. There are cameras in the halls but not pointed into the cells. It’s not clear, but from the reporting it appears the two guards, who were working continuous overtime, were supposed to check on all the prisoners every thirty minutes. That would presumably keep them moving virtually all the time. That would be impossible even for a regular shift. If this is true, then the process was set up to fail.
Other posters with prison knowledge have stated that understaffing is a common problem. My former workout buddy, a prison guard, said it was common.
Yes, working overtime in corrections is pretty much standard. However, when the inmate is some high profile and supposedly attempted suicide previously, standard procedure is changed.