Does the prayer of one who intentionally erred in the middle Tashahhud by adding the phrase "He is one, without partner," then performed the final Tashahhud correctly, become invalid? And is a prostration of forgetfulness required of him?

Your prayer is valid in all cases, because the obligation of the tashahhud in the middle sitting is debated. It is compensated for by the prostration of forgetfulness (sujood as-sahw) according to those who deem it obligatory. If you are a follower (behind an Imam), then the Imam bears responsibility for any forgetfulness on your part, except for the obligatory acts. A follower is not required to perform the prostration of forgetfulness unless their Imam forgets, in which case they prostrate with him.