[X] Merlin
Clairvoyance is op, and his noble phantasm evoking the beauty of avalon. Can you imagine what that could do to some really screwed up capes mental health. To those who have rejected society like Jack, imagine seeing an antithesis to his ideals. Or Panacea's reaction to seeing Avalon, a land that brings hope. The increase in moral to capes during an endbringer attack.
Also merlin ability to be a support for the rest of our servants is indispensable. The ability to heal our servants later is an incredibly invaluable ability, and very few servants can heal on the scale and scope that merlin can, while also being able to dish out as much damage. His item creation might not be insane, but he has Hero Creation EX. All the dumb decisions that Taylor made can be avoided and instead be taught to be a hero of such renown she makes the Triumvirate look like freshly triggered capes. And his Charisma can be used to descalate the scene Taylor no doubt made summoning Merlin.
Merlin may not be the strongest Caster np wise, but he is a candidate for Grand Caster, and will no doubt keep us alive, and in relatively good standing with the authorities. Which is important because the PRT will be the only faction that won't be immediately hostile to us, which gives us time to reach the next summoning. With merlin we can pick riskier low luck servants with stronger nps, as we need a servant who can kill both the endbringers and scion. Avalon on golden morning is invaluable.
[X] Leonardo Davinci
DaVinci maybe riskier due to the start up time that she'll need. Normal tinker start up, but that is made up for in Pioneer of the Stars EX. The ability to influence probability to meet make impossible battle winnable. No battle we have will be just impossible. Even if right now the Triumvirate were to come down on Taylor right now there would be a non zero chance for a fresh Davinci to win. Also Pioneer of the Stars EX counters Contessa.