Thanks for this list. For sure there are some good points in here.
That said, the following is my opinion on number one on your list, and has more to do with 98% of the BJJ training population, and not necessarily professional grapplers on a team (the rules are a bit different when you are getting paid to do something vs paying to do something).
When I started BJJ almost 10 years ago, BJJ instructor loyalty was super important, and 99% of schools were closed off to sharing their "secret" strategies with other gyms. So cross training was a very serious offense - shit was like the Hatfields vs the McCoys. You would literally would have your ass kicked if there was even a rumor that you went somewhere else to train.
Make no mistake, I bought all in on this mantra, too...calling people out and leading the witch hunt. But I quickly saw that it was super developmentally challenged way of thinking about A HOBBY. This isn't feudal Japan, where you kill yourself if you even look crossways at your master...even though mofos like to say "OSS" every third word and try to incorporate westernized pseudo samurai shit into their Insta pages.
Now, in 2017, there shouldn't have to be a delineation between an instructor's school of thought and a "personal belief" of how and when and where you train. Honestly, if your personal beliefs about training don't line up with your instructor's, then find a new school, because it ain't gonna work out in the long run. Trust me on this. Lived it.
I know that I painted with a broad stroke, but really, as a hobbyist, unless there are really good reasons on why your instructor forbids you to train somewhere else, e.g. the other instructor is a dick, the students may try to hurt you, their school charges an exorbitant amount for drop ins etc...then it seems like you are just trying to massage their ego, save their feelings, or help perpetuate some made up beef from years ago. The later are dumb ass jr high school reasons.
But again, great list. And my personal opinions I'm sure make me the asshole here.