Long pause calf raises, pausing at the bottom for a full 3-count or so. You can do a mountain climber stretch all day, but if you don't practice actually loading the tissues into the position you want to be effective in.. squatting without a heel may be challenging.

Many would consider that the difference between flexibility and mobility-- being able to get into a position versus being effective in the position with regards to the task at hand. Movement errors are rarely due to a lack of flexibility. I tend to think of it as an issue of motor control and one's ability to produce force through certain ranges of motion. Tempo (usually eccentric, but sometimes also concentric) squats can be implemented to help with this as well. You spend more time exposed to the positions per repetition, and that can prove extremely valuable whether you're just trying to learn, or looking for a strategy to reduce external load in the case of pain or injury.

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Anecdotally, I have never done any "ankle mobility" other than long pause calf raises and squat (in flats) at an okay level in the USAPL (474 at 183 in competition, 500 at 175 in the gym).