Mercurians - I have never played them, only summoned them, but I think the basics are pretty straightforward. Your
comes from the deaths of units that follow RoK, Empy, and Order, so you want to spread these religions as much as possible. Since you are a warfare-oriented civ, but don't have any particular economic bonuses, RoK's extra gold and Order's maintenance savings can both translate to a larger empire and more
for your ingenuity trait. You already have iron from your palace so you don't need the Mines of Gal-Dur, and they will probably be gone by the time you enter the game anyway. Same goes for Bambur. Arete can help with GP production if you are going for an Altar victory--Kurios build first 3 Altar levels, summon Basium, switch to Basium, pump out the last few prophets yourself. Even so, RoK's benefits are nice but can probably be obtained using it as a secondary religion. Get your 1 stonewarden and switch.
Order's probably a better primary religion than RoK for a few reasons. 1) RoK temples can be built with it as a secondary religion, but Basilicas cannot. 2) Spreading Order itself produces extra military units which can be sacrificed for the cause. 3) Combined with Altar levels, Confessors with Spirit Guide can be used to maximize XP flow to your Angels to build up a crack army of well-promoted troops. 4) Social Order is available if you need it, although admittedly I am not sure if it is better than Arete. With an aristogranarian economy using mines and plains cottages, you can probably get a large enough happy cap using gambling houses. But using Arete locks you out of Apprenticeship which means less starting XP for your angels. And religion provides +5
with RoK/Order/Empyrean temples by itself, which might be as much as you need. Or you might want Scholarship to boost research, etc, etc.
The other choice is Empyrean. This really comes down to a choice of whether you need the economic benefits Order provides, or you need the sheer ass-kicking power of Chalid. This assumes Chalid is still available when you're summoned, but he probably will be because your summoning civ most likely beelined Mercurian Gate because you were playing it. Right?
Leaves and Esus are out as they are for most everyone else, but what about OO and Veil? I am not sure about this, but I think that when YOUR OWN living units die, they also spawn an angel regardless of what religion they followed. Now I believe Basium purges Veil from all of his cities every turn, so you can't follow Veil. OO might be an option though, if you need Cultists for some reason. But I don't think Drowns or Stygian guards turn to angels when they die, so think carefully.