There's only 10 creatures that benefit fully from the cost reduction on Urza's Incubator, assuming you choose Elf. That's not even half of your creatures. I think you'd be better off with Herald's Horn.

Swap Fade into Antiquity for Nature's Claim or Krosan Grip.

Swap Scour from Existence for Beast Within.

Swap Wellwisher for Arbor Elf. Life gain isn't so good in EDH.

Swap Steely Resolve for Eladamri, Lord of Leaves.

Swap Heedless One for Argothian Elder.

Put in Maze of Ith to combo with Argothian Elder. Swap with Autumn's Veil or Hunting Triad.

Swap Wirewood Hivemaster for Fyndhorn Elves.

You might also want to consider putting in Green Sun's Zenith and Worldly Tutor to pick out the creatures you need.

Also, Concordant Crossroads to be able to attack right away once you've assembled your combo.

Selvala, Heart of the Wilds and Viridian Joiner have tap effects similar to Marwyn's, so they are probably worth including.

Sword of the Paruns has the same infinite mana combo as Umbral Mantle, altough without the infinite stats it gives Marwyn.