I notice there aren't any of the general card-drawing supporters (no PONT, Looker, etc.). So you rely totally on Uxie/Seeker for general card-drawing, with no way to just get a whole new hand? Does that really work OK?
Twins is a great card. VS Seeker makes it great too.
Uxie/Seeker is an option, as you have suggested.
Basically when the 1st Gengar Prime is online, it can "tank" for a while given a steady flow of Seekers and a spare line of Gastly/Haunter to Seek into. Whiff, and go for tomb and twins to maintain momentum. Get Uxie/Seekers in your spare time if Gengar is in no trouble. A mix of "any 2" search and some drawing, with a steady "prize" stream, and you will realize additional draw isn't really needed. One energy for a "prize", 2 for two, 130 hp tank, quick heal, you're rolling with just one stage 2 with few energies and supporters. It's quite elementary.
Gengar Prime aims to end the game quickly, so no enduring engine is required. It likes to get Judged, because it uses up most of its resources quickly.
However, in light of this, I noticed a great problem with the Sablelock matchup. Initiative is many times worse than judge, and this is precisely due to lack of draw, and more of search. That fine start you had that will quickly get you Gengar Prime? Flung away because of one heads and your Collector gets sent back. Gengar Prime finally online? Awesome, now meet Honchkrow SV. You can't maintain the hurling, 130 hp Gengar Prime is no longer a tank. So I'm working on that. Whenever I playtest against Sablelock nowadays, I would reach out for Mew Prime and weather the impersonates while i try to get a lead in "prizes", letting the OHKOs be prize exchange than fatal encounters.
Good point you have brought out here.