A couple of comments:
A. I think specification of how divisions are divided in non-top level is required. Specifically: Non-promoting non-demoting people will definitely/maybe/randomly stay together for next season? I think this should be:
1. Decide whether existing divisions will mantain themselves and then assign new people (promoters, demoters, newcommers) in some way, or just take the entire pool of players in a given level and redraw the divisions each season.
2. To draw or assign new players, either use some algorithm considering timezones or pure random (anything else seems unnecessarily convoluted).
I would go for not mantaining divisions, because it is so much easier to process people leaving/joining in between seasons.
B. What happens with the bottom divisions? Each get floor or ceiling of k/nr of divisions, where k is the number of people in the bottom level? That seems like it could give some stupid things like divisions with 2 or 3 people in. I would just have less divisions in the bottom level and try to have 4/5/6/7 people in each division, possibly making only #6 demote from some next-to-last divisions and even have no demotion in some (randomly assigned) if necessary.
C.
2. If a good new player joined, we try to insert at a sensible level
I really dislike this. I understand the reasons for it, but I think it is a too sensitive issue to leave at the organization's will. Basically, I am worried that some people can be easily offended. So, I would either specify a deterministic algorithm beforehand or insert people disregarding "good player" considerations other than last performance in the league. Here is a possible algorithm:
Process levels from top to bottom. If there is a hole and there is a returning player who had that level or higher at the time of departing, put them there. If there are still holes, put highest Isotropish ranking newcomer if it is higher than all possible promoters (all #2/3/4 still available for wildcard promotion). If there are still holes, promote extra people from the immediately lower level (use divisions in random order, first all #2s, then all #3s and so on, and producing new holes to be processed when is that level's turn).
This makes returning players to be inserted at their old positions or worse, prefering promoting existing players if a too high spot is available.
I would still prefer Isotropish (or Goko Pro or whatever other external ranking) to be disregarded after season 1 and just place newcomers randomly. They would raise to the top by promotion soon enough anyway, and makes the league independent from external sources and also the placement algorithm way less convoluted.
D. I think a ruling on Salvager usage and specifically VP counter for online games should be done (i.e., what happens if players do not agree on something). Possibly also for identical starting hands, given that it seems like it would be technically possible in the near future. In all cases, I think both player's agreement overrides the ruling.
E. I think a ruling on a couple of players not being able to play a match should be done. Moreover, I would force some schedule, like you have to play at least half the matches before the last week, to avoid people playing everything on the last day, possibly earning an advantage from knowledge of the current status.
F. I think mandatory/optional/forbidden publicness of results should be explicitly stated. There is reason not to publicly discuss results, because the league is asynchronous. However, I think this is really bad because it stops people from discussing matches online, which is a great thing about playing tournaments.
G. Since several new players and some old players believe that score difference is good to take into account, I would explicitly say (even if it is redundant) that the final score does not matter. Also, I would say tied score counts as win for the player who played less turns, if such player exists (because "tie" in the rules could be interpreted as tied score instead of tied game by the rules of Dominion). I would just say "shared victories" in the rules instead of ties, to make it clear that the official rules apply. Technical thing: Can we just award lone victories 2 points and shared victories 1 point? I don't like the halves if we can avoid them.