Is it valid to say "التحيات لله والصلاة" (All greetings are for Allah and the prayer) instead of "الصلوات" (all prayers) in the final Tashahhud, given frequent intrusive thoughts (waswasah), and does this entail repeating the prayer or incurring sin by repeatedly redoing it?

There is no harm in saying "Al-Tahiyyat Lillahi, wa al-Salat" instead of "Al-Salawat," whether in the first or second Tashahhud, as both have been reported in the Sunnah. Your prayer is valid, and there is no need to repeat it, as that would be an act of extremism and excessiveness in religion, which the Messenger of Allah, peace and blessings be upon him, forbade.