Say, Dode, this occurred to me. On page 12 you told Koadah:
"Regular" to me meant "on a schedule", i.e. regularly. Scheduled leagues are specifically mentioned in the rulebook (LRB6 page 31):
Your quote is somewhat out of context. It is in the middle of a section with examples of how to resolve who progresses to an end of season tournament. First paragraph goes "one method....", second goes "another method" and the third one goes "finally, a League Commisioner can decide to run the regular season as a League with scheduled matches...".
So it is not on an exclusive list of allowed League structures.
Earlier on the same page it also states that: "He (the League Commissioner) will need to let his coaches know how long the season will run for, and how many end of season tournaments will be run."
A dozen of matches is suggested. But not a rule. It would be perfectly legal for a commish to say: 30 years.
No primacy is given to fixed Schedule Leagues at all.
In fact, on the top of the next page open Leagues are mentioned implicitly, when it talks of people dodging games - which is not possible in a scheduled League.
So open Leagues are in the rulebook every bit as much as scheduled Leagues.
I don't know if you were playing BB back then, but in fact open Leagues was one of the big new Things introduced in 3rd ed., presumably because scheduled Leagues have a tendency to collapse when one or more coaches quit the League.
That said - you are absolutely correct that according to the rules an open League must allow challenges and challenge refusals/swaps.
Cheers
Martin