red alerts
Good preparation, but a number of red alerts were ignored, here.
"I felt some resistance and pulled gently since the RJ45 jacks on Cat 5 cables can take some strain without damage, and all the cables were out anyway..."
Red alert one: if there is resistance and you can't explain it right now, stop, and only restart until you're confident it has been dealt with.
"The Data Centre helpdesk team had not shared their step ladder due to "health and safety""
Red alert 2: if you don't have every tool you'd potentially need, refuse the operation.
"Colin hadn't been given the key to get into the rear of the rack."
Red alert 3: like #2
Transformation is about sorting out years of mess, so you'll always step onto some crazy temporary "fix" from a long time gone permie or contractor.
It's really like walking on a mine field. Every bizarre thing counts.