Seems obvious to me, so someone correct me if I am off base here.
RG hired KD at the end of February, his pickings were rather slim, went after a few guys he wanted and they all passed (turned them into raises, or wasn't the right time). Even those who he went after had some rather big question marks around them. They were stumped, and as RG and LC talked about, who do we talk to next. KD fell into their laps as someone who was in theory a low risk and high likelihood of landing. KD (more likely his agent) turned that level of desperation into a very lucrative contract since they had all the power.
I was thinking at that time they should have just named a interim coach for the next year, then started the search all over again during the normal cycle. In hindsight that would have likely lead to a Chiv IHC, and would have likely ended up something similar to Embree.
RG didn't work miracles with Mel leaving, but with what he was dealt, I struggle to fault him too much looking back.