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1. Heather Hoover was a co-worker and friend of the family, dating back to when they were neighbours. She probably had kids the same age as the Jessop's kids, IMO that, and the work relationship, would explain the neighbourly visits.
CH was an oddball, not very friendly. Heather evidently tolerated him as the husband of her children, but later divorced him.
"From (Heather’s) description, he wasn’t a nice person. He was just a bit of a mean, nasty person. He wasn’t home a lot and spent a lot of time out of the house. He didn’t spend a lot of time with the kids."
'He was a bit of a weirdo': An image of Christine Jessop's killer, Calvin Hoover, takes shape
IMO the connection was between the Jessops, and Heather and her kids, CH was probably just ignored. Therefore they wouldn't connect him with the crime because a) he was Heather's spouse and she was a good friend ergo, he must also be a good friend, and b) he lived fairly far away and was supposed to be at work when the crime occurred.
2. IMO we don't really know the full context of how his name appeared in the file, or what follow-up was done. For example, as has been speculated, Mrs. Jessop may have commented about her husband's work colleagues coming into eg, the basement to take supplies if they were working in the area. Then maybe a lengthy list was compiled of all those colleagues, which included CH's name. But perhaps the list was just used to check for people who were no longer employed, the assumption being they were looking for someone who was holding Christine alive. Perhaps the notes were in some seargeant's notebook, and there were dozens of boxes that were just too disorganized for the newly struck murder investigation to try to wade back through them all.
I think police have long since learned how to better conduct missing children investigations, IMO trying to go back and pin blame on one or other particular police officers' errors is not a worthwhile excercise.
For eg, why aren't there calls to go back and scrutinize every note of every police investigation into the Golden State killer. How could they have missed him, etc, etc for every cold case finally solved through DNA geneology.