Can players command Aniimo from the sidelines?

Staying in human form is a full combat stance, not a fallback for when you are out of options. Your input is the ability calls: which skill the Aniimo uses and when. Your own contribution on top of that is support buffs, not attacks.

What you give up is direct control. You are not steering the creature's movement or timing its dodges yourself, so a fight run this way lives or dies on ability selection and on keeping your buffs up.

Twining is the other way to fight

Twine mode inverts the arrangement. You directly control the Aniimo's actions, movement and combat kit, and twining hands you its movement style, body, powers and abilities. Merging gives you access to the creature's powers and combat abilities as your own. Transformed, you are the one handling skills, dodging and dashing, moment to moment.

Transforming also gives you a separate health bar in battle, which makes this more than a control preference. Commanding from human form leaves that second bar unused.

Carrying an egg does not lock you out

If you are twined with an egg, you can untwine to fight and retwine once the fight is done. The egg does not pin you to one stance for a whole run.

Not settled: It is not settled which stance is stronger in a given fight. Human form fights through commands and support buffs, a twined Aniimo fights with its own kit and its own health bar, and the two have not been measured against each other.