Litebus, the MEL as the name suggest, is a List only. It doesn't contain the diagnostic procedures, that are specified in other manuals and are part of an Engineer's experience and trade. To restate in other words, the MEL is not a bible to which adhere when mantaining an A/C. and mere observance of it does not gurantee that one have done a good job.
At the end of the day, they dispatched A/C with all the systems depending on R2-5 in ground mode, inoperable.
When I troubleshoot electrical in my car (it's a bit old so sometime it needs that), I use a schematic and try to understand all the reasons and ramifications. I know it's a oversimplifying comparison, but when they did started work on the the A/C, either did not used a schematic, or failed to interpret it properly.
I'm convinced that you can ask any certified engineer on the MD-80, and he will tell you that the expected professional standard would have been either diagnose the real fault, or not release the plane for flight. Remember, an alternate machine was avaialble and it had been considered already.