A small point - do you still think Woodfall Primus is a better choice than Terastadon for a dread return target in dredge? Otherwise, I think the turtle dredge "shell," if you will, is a great way to approach the deck. Has serenity typically been castable in your post-board games, and has the maindeck darkblast ever been particularly relevant over, say, another golgari thug or a second Dakmor?
I tested with similar decks, so I think I can answer two of your questions.
Woodfall Primus can lock the opponent out of the game. Terastadon doesn't. It gives them blockers and blockers that make winning much more difficult, which gives the opponent more turns to recover. I personally do not think they are comparable because of that. The extra Zombie Tokens is also helpful.
Darkblast is often a very useful card that has won me multiple games. It kills Bridge from Below in the mirror by killing your own Ghast or Narcomoeba. Against Oath, it can kill Orchard tokens if that is ever important. It kills Bobs and is the only maindeck answer to random Gaddok Teegs as well as killing most of the creatures in Fish.