Does intentionally thinking during prayer, and intentionally reciting continuously until one's breath runs out, and the consequent unintentional mispronunciation of some letters, invalidate the prayer?

You must strive for khushu' (devotional humility), as it is the spirit and essence of prayer. Prayer is not invalidated by the thoughts you mentioned, nor by involuntary speech, because prayer is only invalidated by intentional speech. As for the speech of one who forgets or makes a mistake, it is excused, based on the Almighty's saying: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended" and the Almighty's saying: "Our Lord, do not impose blame upon us if we have forgotten or erred."

So, continue to strive, and your prayer is valid, Allah willing.