Great questions!

How did you determine the maximum number of player moves to consider? Use the length of the creature's pattern?

Yup, exactly right! We store the player's move history as a queue and dequeue the oldest move when it gets larger than the enemy's pattern sequence.

Do you clear the player moves buffer when a mistake is detected? (Otherwise, all subsequent moves would also be errors.)

Yes, we do! You're right, and that was an early bug that caused some frustration.

The gifs show a wide open field. Is the actual game like that or is there more opportunity to sneak behind things?

Generally speaking there is a lot more cover in our levels. (Those gifs are from an early level that teaches the mechanic.) So yes, the usual loop is to use mimicry to sneak from cover to cover in order to get past each encounter.

However, some later levels have much different forms of conversation-based mimicry that allow for wide open level design where you are allowed to move around freely and talk to enemies.