#6 seems a bit misleading. As others have said, the original Judeo-Christian Sabbath, the day of rest, was definitely Saturday, the seventh day. (That is, in Jewish terms, from Friday sunset to Saturday sunset.) The Christian day of worship was apparently already changing to Sunday, the first day, by the 2nd century, when Constantine made it the official day of rest in 321 AD. The first day was the day of resurrection, or rather, the day the tomb was found empty, so it symbolized renewal and a new beginning. The change also apparently served a unifying function, both to set the new Roman week apart from an earlier Roman 8-day week (! that was news to me) and to distinguish Christianity from Judaism.
So yes, normal calendars in English do traditionally begin with Sunday, and it is definitely the first day of the week. However, in recent years I think I've seen some business-oriented calendars that start with Monday, probably so that the week and weekend are more clearly separated. There's no law against it, it just looks funny.
Either way, the ISO surely doesn't have any jurisdiction over English-language publications, and if it tries to promote nonstandard 'standards,' it will probably only discourage people from adopting them voluntarily, at least in contexts other than scientific or technical.
Some of which I probably said in the previous discussion linked in #9, or in this one:related discussion: What day is the first day of the week - Sunda...
Sorry to repeat if so. (-: