What is the ruling on the prayer of one who intended performance, then during it spoke to himself about changing it to a missed (qada') prayer, then settled on the intention of performance, knowing that he suffers from severe intrusive thoughts (waswas) regarding the intention in prayer and ablution?

If one intended to perform a prayer as a current performance but it turned out to be a make-up (qada'), or vice versa, their prayer is valid and not nullified by that. The advice for anyone afflicted with obsessive whispering (waswasah) concerning intention or anything else is to completely ignore and disregard it, and not to pay attention to any of it, no matter what the whispering suggests to them.