When making mayo, I drop the mustard and the vinegar. I normally use vegetable oil - it’s cheaper than olive oil. I don’t like the taste of mayo made with sunflower oil. And it keeps for a week or more.
And I add lots of garlic. 
Correct. Put an egg yolk in a new bowl and slowly add the curdled mixture while beating all the time.
I’ve never made mayo myself, but when I get around to it, I’ll do it just like this guy. Looks easy, with no tricks up his sleeve. 
I have. Every stupid time. Cream the butter and sugar, check. Room temperature eggs, check. One at a time into the Kitchenaid, beating away merrily with the whisk attachment, dammit, dammit, dammit. No more smoothly emulsified mixture, it looks like little bits of butter suspended in…something.
I hate wasting food so I just ignore the fact and add the flour or whatever next. But I keep thinking, no, I don’t think that’s right. Maybe, no, that definitely isn’t right.
Cake usually tastes fine, I mean, who complains about free cake?
I still wonder what the heck I should’ve done.