Even on 1), the first day of the week being Sunday (US) and Monday (Europe) screws me up constantly, as someone who is in both worlds now.

I often pick the wrong date for meetings because I'm assuming next Monday will be in the first column, not the second. As dumb and simple as it is, when something has been drilled into you since about age 3 its tough to get in the habit of double checking it when it starts becoming inconsistent.

It depends what kind of business... there are all sorts of weird and wonderful accounting / financial calendar inventions like the 4-4-5 calendar, fiscal years ending in June, bond day count conventions like 30/360, etc.

I do take your point though. I've always found it quite surprising just how universal various arbitrary/western things are like, as you say, the western gregorian calendar, seconds/minutes/hours, SI units for measurement. In a world with so many unintelligible languages and disagreements, you'd think there would be more folks out there doing, I dunno, a 10 day week, or some non-hour/minute system, etc.