One thing I would need to mention is that the significance of 「おやつ」 as a custom has decreased dramatically since Edo period. Japan had largely been a two-meal-a-day nation until around the middle of Edo. There was no such thing as lunch for many people.
Naturally, you would become hungry by around 2 o'clock in the afternoon, which was called 「やつ/やつどき」 by our old system of telling time. So, you would take a break from work for a snack and tea. It was named 「おやつ」 and it was more of a necessity than leisure back then. It was, in a way, more like a tiny meal.
In our time, there is lunch and therefore 「おやつ」 is much more optional among many. (Or at least we could wait till 3 o'clock instead of 2 like we used to, thanks to lunch.)
- White-collar workers never do "おやつ"? They might use it as euphemism for briefly standing and stretching in mid-afternoon?
Many do, but they do not do it together at a designated time. White-collar workers often keep snacks in their desk drawers and nibble on them at different times of the day.
- Do blue-collar workers do a real "おやつ" and rest / eat in mid-afternoon?
Yes, they tend to do so together and at a designated time (usually 3:00 pm), which is the huge difference between the two collar colors.
- Etymologically speaking, a farmer's お弁当 was his おやつ in 大和 Japan?
I do not know the real answer to this, but I suppose one could say that. Traditionally, farmers have had more food available to them than other types of workers. If the content of their おやつ had been more meal-like than that of others, that would not surprise one.