What is the ruling on saying "Allahumma salli 'ala Muhammad" (اللهم صلْ على محمد) with a sukoon instead of a kasra in the tashahhud, and is this permissible due to the fear of waswasa (whispers/obsessive thoughts) regarding pronouncing it with a yaa'?

You must beware of obsessive whispering (waswasah), turn away from it, and not succumb to it, as it leads to great evil. Pronounce the words correctly, and do not doubt the pronunciation afterward. Recite "Allahumma salli 'ala Muhammad" with a kasrah on the lâm, without a yâ'. Do not pay attention to Satan's whispering that you pronounced the yâ', and do not vocalize the lâm as a sukoon, for that is surrendering to waswasah. Your prayer is valid if you do so.