Its not just a double Entomb tho, its 2 in hand (that a majority of the time both can be played) and a double Entomb as you put it. I personally would try 1 Realms over 1 Ancient Grudge in that list, and just see how it works out. If Im wrong and its garbage, that is fine, but I think it will be very useful in a lot of scenerios.
Ancient Grudge needs to be a 3-of minimum. Realms Uncharted doesn't solve the problem Ancient Grudge solves against decks relying heavily on artifacts and especially artifact mana. Ancient Grudge is vital both as a way of taking out Workshop's creatures directly or taking them out indirectly by destroying the mana they'd use to pay for The Tabernacle at Pendrell Vale. Realms Uncharted also does nothing to speed up our clock or slow down the clock of Paradoxical decks. Taking them off one mana source during the casting of Paradoxical Outcome, especially Mox Opal, nets them one less card and possibly the mana to chain multiple together. Hitting Time Vault after Voltaic Key targets it is wonderful too. I wish I had room in the maindeck or sideboard for the 4th Ancient Grudge, but I need to hit other permanent types. My hope is always that Null Rod functions as the 4th Ancient Grudge when it matters.