I think the must have 8 in a scrum is badly worded.
The law makers are assuming that games have started with a full compliment of players.
We have a regulation of at most one player difference at the lowest levels (4th division for gents; there is a premier which is about L5/L6 by UK standards).
I would interpret it as "when scrums go uncontested, if one team has fewer players than the other, the numbers in the scrum must still be equal".
Youth regulations are also with 7 each, drop the number 8, with 6 each drop both flankers, and with 5 each no back row, but equal numbers.
So if one side turns up with 10 and the other with 15 or more, and the side with just 10 doesn't want to borrow players (yet), they will be playing 5-man scrums, but if it is relatively friendly and everybody wants a game, and it starts off 12v13, then I would (ok, might) make the decision that scrums are six aside from the start, and stay so if scrums go uncontested for any reason, even for adults.
I don't think you can really encode that in law, and the document in post 1 is the "made easy" version.