That only works if those mana sources tap for the colors of mana you need.
2 Cavern + 4 Vial cannot cast
Swords to Plowshares or some SB cards. They only help you play high CMC spirits (quantity of mana vs quality of mana). That's where I could see it running into problems. Lack of access to basic Plains and colorless flood may be where it's stumbling. Changing the fetchland configuration away from 4 Delta or 3 Misty in the above lists (Island-heavy) may help improve the quality of mana sources. You should be able to StP anything when you're only on 2 lands, fast enough to interact with Marit Lage & Murktide, without worrying about mana denial keeping you off it.
Good point. Maybe you don't need as many basic Island then and can run more Tundras there (for Daze). You won't need more than 1 basic Island in play most of the time.
And if you're losing to Stifle, extra duals are better than off-color fetches. You don't have Brainstorm or Delve or Escape or other payoffs for fetches, so non-UW fetches are much less good for you than for most decks.