There's a whole lot of fantasy in there, so here's mine.
Leavitt hadn't run a defense in many years. He had not seen modern CFB offenses EVER. It was MM's defense at least as much as it was JLs'. I've heard some say it was all MM's. It was MM's secondary (that I'm 100% sure of). Leavitt molded the front 7 and was a supreme motivator and good game day coach. Overall, certainly a good guy for the job.
Because Leavitt was always trying to position himself to be the HC, it was always about "him" and it rubbed every staff member the wrong way from day 1. That's why you didn't see MM deflecting attention to JMFL. His ego had already outgrown Boulder.
Leavitt may be a supremely great DC, but I think it's less than a given fact at this point. 2018 should show us a lot, as Oregon has recruited MUCH better talent over the last 5 years than anything he worked with at CU. Much better. Perhaps a better metric is if the year 2 Oregon D doesn't eclipse year 2 he had in Boulder, it wasn't just him?
Oregon's defense pre-JL was a mess. But it wasn't because of talent. I do acknowledge they had some holes up front. A "competent DC" should have seen great improvement in 2017 and again in 2018. If they are not a top few in the league this year, then I'll have to wait on the JL is the greatest for a bit longer. And he might be, I just think you are jumping ahead of the facts.
The ultimatum B.S. re: Chev is sort of silly. MM had already demoted Lindgren when he hired Chev. He gave Chev the co-title even though he had no experience as a OC. It's pretty plain to see that MM had decided he needed more than Lindgren was giving.
I encourage you to use paragraphs when beginning new thoughts.