Is the wording of the Tashahhud mentioned in the question such that it invalidates the past prayers of the questioner, and must he make up past prayers due to his doubt about pronouncing some words in the Tashahhud?

The prayer is valid and does not need to be repeated, because the final tashahhud is a matter of scholarly disagreement. It is a pillar (rukn) according to the Hanbalis and Shafi'is, and a sunnah according to the Malikis and Hanafis.

The fatwa (religious edict) that invalidates the prayer of one who erred in the tashahhud is based on the opinion that it is a pillar. Since the questioner is from a country where the Maliki madhhab (school of thought) predominates, he may act according to the dictates of his country's madhhab, and his prayer is not invalidated by an error in the tashahhud.

The preferred form of tashahhud according to the Malikis is the tashahhud of 'Umar ibn al-Khattab, may Allah be pleased with him: "All greetings are for Allah, all pure deeds are for Allah, all good things, all prayers are for Allah. Peace be upon you, O Prophet, and the mercy of Allah and His blessings. Peace be upon us and upon the righteous servants of Allah. I bear witness that there is no god but Allah, alone without partner, and I bear witness that Muhammad is His servant and His messenger." And one says "All good things" (الطيبات) only, without "for Allah" (لله).

And sending blessings upon the Prophet, peace and blessings be upon him, after the tashahhud and before the supplication, is sufficient in any form, though it is preferable to stick to the form authentically transmitted in the Sunnah.

The questioner should ease his mind regarding the repetition of the tashahhud, as that is an indication that waswas (whispers/obsessive thoughts) have taken hold of him.