Is the ghusl of a non-Muslim—whether an infidel or an apostate—who performed ghusl solely with the intention of removing janabah after uttering the Two Testimonies sufficient for the ghusl of entering Islam, and is he thereby pure and are his acts of worship valid?

What we choose is that an unbeliever is not required to perform ghusl (ritual bath) due to disbelief. Rather, ghusl becomes obligatory upon him if he is in a state of janābah (ritual impurity requiring ghusl). So, his ghusl is for the purpose of removing janābah, not for entering Islam. Therefore, the ghusl of this person is valid, and his acts of worship are valid, even if he did not intend by his ghusl the ghusl that is required from someone who enters Islam.