Hello, thank you very much for the thorough review, I really appreciate it!
You are completely right, some of the obscurity is "by design" because I wanted the player to try and understand the mechanics as they play the game, but I realize that overall everything is just too much obscure. I really should have explained how the fight planning works and I should have added more information on the fight outcome screen.
The selected attacks are all used in the battle, and there is no downside selecting more than one attack (though selecting an attack for which the opponent has protection penalizes you in battle), apart from spending more precious gems. The zero cost attacks are weaker than the attacks which cost more, so using the zero cost attacks on stronger opponents (higher levels) is not as much effective as using the attacks that costs more gems.
I realize this information is really missing from the game. I should have explained this and other concepts at least on the game page :D
Cheers!