Great thread, Madgran! And lots of good points here, IMO.
Just want to add that I think SOME GPs may feel they're "walking on eggshells" b/c of a certain mindset. They approach their AC and CIL as if they are "supposed to" evaluate them, their home, and their children, and, no surprise, almost always find some fault somewhere. My MIL was like this, always entering our home or greeting us w/ a "critical eye," searching for flaws. Then when they start complaining/criticizing and get told off, they feel as if, "I can't say anything,' etc. (NOT saying that's true for all GPs or any of the GPs here.) These kinds of MILs/GPs feel "freer" around other people b/c they don't approach them the same way. But they don't realize it, unfortunately.
Also, sometimes, I've heard people say that who look at every incident as totally separate. So if, say, DIL jumps on them for letting the GC stay up late on a sleepover one weekend, and then for giving them some extra treats on another one, the GP sees it as 2 separate issues. But, of course, it's really one - breaking the parents' rules for their kids. (Again NOT saying this is always the case.)
I get that some people just explode randomly and I'm so deeply sorry that some of you have had to experience that, especially as children.