I've been experimenting with a Heartless Summoning deck, although I have been focused on trying to get a combo going rather than using emerge costs. It's been a fun deck to build and I think Heartless Summoning has untapped potential, although of course one issue with these decks is what to do when you don't draw it. There are tons of great combo possibilities, I love your idea of using the emerge cards with flash to get extra value out of evoking Mulldrifter or Shriekmaw!
It was built around Mindshrieker but ended up using some emerge cards to take advantage of the high converted mana cost with an alternate cost. Probably not too competitive, but a ton of fun in playtesting.
Turn 1: On the draw, draw a card. Play no cards, end turn. Discard a Shadowborn Apostle to get down to 7 cards. Pay 2 life, cast Surgical Extraction to remove your Apostle's, then play another extraction to remove the other Surgical Extractions. Now your deck size is as small as you want.
Whenever enchanted creature deals combat damage to a player, that player reveals his or her hand. You choose a nonland card from it and exile that card.
When Tidehollow Boon leaves the battlefield, return each card exiled with it to its owner's hand.
For reference there is an old thread dedicated to a Heartless Summoning + Myr Retriever combo.
Regarding splashing white, I think Athreos, God of Passage would have a strong synergy in a deck like this.
Shriek Softly and Carry a Big Stick
It was built around Mindshrieker but ended up using some emerge cards to take advantage of the high converted mana cost with an alternate cost. Probably not too competitive, but a ton of fun in playtesting.
Nemesis of Mortals would also work along this line.
Turn 1: On the draw, draw a card. Play no cards, end turn. Discard a Shadowborn Apostle to get down to 7 cards. Pay 2 life, cast Surgical Extraction to remove your Apostle's, then play another extraction to remove the other Surgical Extractions. Now your deck size is as small as you want.
Heartless Summoning + Heartless Summoning + Ancestral Statue
Rakdos, Lord of Riots + Ancestral Statue
Aura - Enchant Creature
Whenever enchanted creature deals combat damage to a player, that player reveals his or her hand. You choose a nonland card from it and exile that card.
When Tidehollow Boon leaves the battlefield, return each card exiled with it to its owner's hand.