I am a writer for Black Magic Gaming, color commentator for the Mana Drain Vintage League, and current Vintage MTGO streamer at https://www.twitch.tv/vaughnbros
Mark's list was fairly stock, at the time, outside of the sideboard if I remember correctly (that is to say it was some variant of Always Sunny Dredge). This was all prior to you and I developing the transformational board though since I recall I was playing a very early iteration of my creature transformations (aka bad BUG fish). So it was quite a few years ago. Its possible that the meta has shifted to make this more playable, but seems unlikely to me as the format was super slow comparatively, which is why my plan, at the time, of Dryad Arbors + casting stuff like Golgari Thugs could get there in game 2/3s.
I've seen a certain World Champion Dredge player try this board configuration before with the Harbinger, and to say the least he didn't keep running the deck very long. Its too much set up just to kill 1 anti-hate piece. The format is so much faster now since I've seen that list too. I think you just have to take the gamble with a SB like this and hope that your opponent doesn't pull out something unexpected.
Muldrotha is definitely playable, but not really in these lists. There is a really nice infinite loop with it, lotus and animate dead that allows you to recur all your non-land permanents if I can ever make it to a paper event (or MODO actually learns how to fix the no looping issue).
Its an okay board plan for a Manaless Dredge deck. Having uncouterable answers to hate is great. On the other hand, having to guess exactly what hate pieces your opponent is playing is not great.
Just a note on one of the comments, dismember is also a poor answer against Lavinia because you need 3 lands in play to cast it. Reality is if your opponent casts both Lavinia and Contaiment Priest before you do anything, it was a pretty rough game. Spinning Darkness is superior as it only takes 1 card to remove the Priest, not 2 that dismember requires.
Some random thoughts on my graveyard cards...
I think a creature with Dredge would make more sense if you wanted to just slightly bolster the GY plan from the deck. Dredge creatures give you a similar effect of Squee to guarantee a creature to pitch to Survival, while potentially also hitting a Vengevine / Wonder / ect. Golgari Brownscale is a non-traditional one that can give you some life points, and as a result stem the bleeding against other aggro decks. Or the classic, Golgari Grave-Troll gives you an endgame finisher.
A heavier splash of bolstering the GY plan, I've tested a bit, but haven't found a sweet spot. Hermit Druid is nice at times, but too inconsistent (adding the typical package of DR + Target + Therapy is too much). Gravecrawlers are nice with an active Survival to trigger Vengevines every turn, but otherwise are pretty underwhelming, I think you might need to find some other zombies that are good (I tried Fatestitcher and the Embalm Cat, but they are not good enough).
As someone who plays a lot of Dredge, but also dabbles in every other archetype, I must say that the most important thing to choosing a hate piece is really how it synergies with your deck. Some decks even want to run these cards main deck, and not just leave them to the sideboard. This usually leads decks to play a 4-of since most decks have 1 hate piece that is just superior for their deck list. For those that don't pick anti-hate that synergies with their deck, they run the risk of jeopardizing their own gameplan and thus playing into the Dredge players' benefit of both of you having worse decks in game 2/3.
For example, just playing Leyline because it theoretically can hurt Dredge the most is not a good reason to run Leyline. If you're on a Xerox deck that wants to filter through its library and now you have a bunch of bad cards in there. If you're on a Outcome deck that wants to activate its mox opals and cast outcome to bounce as many permanents its also not a great choice because Grafdigger's, Tormod's and others are artifacts and are better at being bounced. If you're on an aggro deck, the damage difference between a swinging Yixlid Jailer / Containment Priest and a Leyline of the Void can also not be palatable.
Its a combination. There isn't a Vintage pro tour event so obviously the pro players impact is much lower than in formats where there is a pro tour. As such, I think most people are playing the format for fun. That doesn't mean that they aren't going to be competitive and try to win though.
There are some players that will switch decks often, and constantly trying new things to get a leg up on competition. There are other players that will run the near same 75 for years, and/or run decks that they know are sub-optimal just because they find them fun.
There are areas of the world, where tournaments are frequent and predictably held (like online and in the North East), and there are areas, where they are not.
The format doesn't rotate, or have frequent bans (like Modern + Standard) so the format is "solved" to some degree in the same way that Legacy is "solved". We obviously still have the occasional restriction, or major printing that will change the format.
Wispmare counts as a creature for your Hulk and Tree. If the goal is to build around these two cards, it seems to me that you'd want to maximize the number of creatures in your deck.
Why does countermagic fit in this build? Doesn't that reduce your creature count over something like anti-hate Chewer+Wispmares? Or some other creature based set up?