I really like the idea of what this deck is trying to do.
Descendant's Path is a really awesome way to cheat out Emrakul, the Aeons Torn because it actually triggers his extra turn effect.
I think you should have more than one copy of Emrakul in the deck, he's your best win condition, the last thing you want to happen is to draw your 3 card combo in your opening hand then top-deck Emrakul before you're able to play the other pieces of your combo.
One thing about playing a combo deck is increasing your chances of drawing all the required pieces and enabler cards that help pull it off faster. In this case you I'd recommend 4-of Mutavault, Descendant's Path, and Congregation at Dawn. You've already got an 8 pack of one-drop green enablers in Birds of Paradise and Noble Hierarch to help you get to 3 mana on turn 2. So that is pretty good, you may want to consider 1 more mana dork, perhaps Avacyn's Pilgrim to help with the white mana required for Congregation at Dawn.
Also to this end I think you need more land and besides Mutavault nearly all of it has to be able to produce Green mana on the turn it is played.
If you think about your combo is mainly centered around 2 cards Mutavault and Descendant's Path. Congregation at Dawn is an enabler that seals the deal.
Under an ideal opening draw our combo wants you to:
T1: Play Green Mana source, Play Mana Dork
T2: Play Mutavault, Play Descendant's Path
T3: Untap, Upkeep, Descendant's Path triggers, while it is on the stack, activate Mutavault with it's own colorless mana, cross fingers, reveal top card and hope it's a powerful creature. If not draw that card. Play a third land. Cast Congregation at Dawn arrange top of library with Emrakul on top. End turn, hope for no mill effects.
T4: Untap, Upkeep, Descendant's Path triggers, while it is on the stack, activate Mutavault with it's own colorless mana, cross fingers, reveal Emrakul and cast him for free, get extra turn and probably win the game.
As you can see, extra copies of Emrakul increase your odds of unleashing your win condition sooner if a copy just happens to be on top of your deck at the beginning of Turn 3. It also helps protect against drawing your only one on Turn 2 making your Congregation at Dawn less useful, still good considering the other powerhouses you are packing but Emrakul's extra turn is the real win condition.
Now we should look at other Modern cards that do the same or similar things to our combo components. Unfortunately that's pretty tough for Mutavault and Descendant's Path. Congregation at Dawn is a little easier to find a similar effect, even if it's not as powerful.
However if we focus on Emrakul and not the other powerful creatures you have in the deck we have alternatives to Mutavault. We just need an Eldrazi on the board when Descendan't Path triggers. There's a few 2 drop spells that are Eldrazi Creatures or spawn Eldrazi tokens. Nest Invader is probably the best bet for the colors you're using. Definitely slower than Mutavault but faster than having to tutor for Mutavault and play it.
Now we increase our odds of our win-con because we're not totally dependent on drawing Mutavault to cheat Emrakul onto the board.
Other options to help us get Emrakul on top of the library, especially if we play 4 copies, possibly could include Index and Mystic Speculation. There may be others that would be good.
So those things help us get to the Emrakul win-con a little more reliably.
Now we have to consider what is our opponent going to do or try to do to shut down our strategy and how do we stop them.
1) Remove the Eldrazi or Mutavault after you activate it but before Descendant's Path trigger resolves stopping your from casting Emrakul or other creature for free.
Potential answers to this are counterspells. For this purpose Dispel would be particularly efficient but any one-drop counter that can target an instant spell can work. Swan Song is another good option. The 2/2 flyer they get will likely be annihilated before it can block Emrakul. Avoid Fate is another good option.
2) Destroy/exile or even counter Descendant's Path before we hit with it.
This is trickier than the last example. If you cast Descendant's Path when you really want to cast it you should be tapped out. Making a counterspell pretty much impossible. Obviously in games 2 and 3 you'll be better able to anticipate how your opponent might do this and can wait a turn or two to cast Descendant's Path to make sure you have the mana available to protect it. Pact of Negation doesn't help because you'll only be untapping 3 mana sources next turn so wouldn't be able to pay 3UU during your upkeep. Not having a good answer to this is probably your deck's greatest weakness.
3) Removal of Emrakul. This is usually pretty difficult to do but there are ways to do it. Retaliate, could get him he'd have to have the mana available and float it during the combat phase before annihilator probably forces him to sacrifice his lands, then use the floated mana at the end of the combat step to cast it, destroying Emrakul. Your opponent is still pretty much screwed without anything in play but it at may slow you down a turn depending on how you setup your top of the library.
Your opponent could also Celestial Flare or something similar so make sure to attack with all your creatures during the extra turn so you can sacrifice one of them instead of Emrakul.
Extremely modified version I put together that does most of the things I talk about above:http://deckbox.org/sets/586967
Obviously it got a bit more expensive but it should hit your win-con more reliably and has a means to defend it.
Hope this helped.