Just a sidenote on Gemini, I don't believe that the vanilla moveset of "3 range rook" is worth 8. Probably around the lower end of 7-8, especially as Crusader and AirElemental are basically strict upgrades to that.

I don't have a problem with LifeStone reviving things. I'm thinking of a design in our game that does the same but is more granular with costs (so you could revive a number of different units, or one big cost one). Though this is even more difficult to balance and may not see the light of day (because of the idea of "unit advantage", the idea that units may have a different extra innate cost than their number such that, say, two WaterElemental for a Rook is NOT an equal trade - for example, Pawns in this game are 0 but not literally zero value). To make a properly balanced unit like the one I mentioned, you would basically have to figure out that number and incorporate it into the design, or rebalance the game so the piece costs make sense with the unit advantage idea.

I feel like that interaction is pretty intentional, at least the Phoenix LifeStone part. Gemini's value was probably pulled out of LifeStone range for that reason, as you could do it with Gemini too. But there's a devbot army that uses 4 Phoenix+++'s and spams LifeStone+++.

Basically, currently I think LifeStone is really bad design right now, but the idea of reviving (which is really broad) can be quite good.