If I play a card like Yone with 5 might from hand, can I trigger Grand Duelist?

Regarding your question about whether playing a unit with 5 Might (like Yone, Blademaster) triggers an ability that cares about a unit "becoming Mighty," such as the ability found on Fiora, Grand Duelist:

According to Rule 709:

"A Unit 'becomes Mighty' at the moment its Might changes from being less than 5 to being 5 or greater."

Because a unit played from your hand enters the board already at 5 Might, its Might value does not change from less than 5 to 5 or greater during the process of entering play. It simply enters the board as a Mighty unit. Therefore, it does not satisfy the condition of "becoming Mighty" as defined by the rules.

Summary of the interaction:

• Entering Play: When you play Yone, he enters the board with 5 Might.
• Trigger Check: Because his Might was already 5 upon entering the board, he does not undergo the transition required to "become Mighty."
• Result: Any ability that specifically triggers when a unit "becomes Mighty" will not trigger upon Yone entering the board.

Note: I could not fully verify this ruling against the retrieved rules.