At what point at the start of the game do you resolve the effect "at the start of the game, play up to 1 {Mary Geoise} type Stage card from your deck"?
Effects reading "At the start of the game" are resolved after shuffling your deck and revealing your Leader card, after determining who goes first, but before drawing your opening hand. In this Leader's case, after deciding who goes first, the player using this Leader decides whether or not to search their deck and play up to 1 {Mary Geoise} type Stage card. If they do, they search their deck for the card, play it, and then shuffle their deck again.
If both my opponent and I are using this Leader, which player resolves the effect, "at the start of the game, play up to 1 {Mary Geoise} type Stage card from your deck" first?
If both players need to resolve this "at the start of the game" effect, the player going first plays their Stage card first, followed by the player going second.
What is meant by this Leader's attribute being "?".
It means that this Leader's attribute is not any of "Slash", "Strike", "Ranged", "Special", or "Wisdom".