What is the ruling on prayers if "التحيات لله" (All Salutations are for Allah) and "النبي" (the Prophet) are recited with a sukoon on the yaa (ي), given that the original vowel for both words is a kasra (ــِـ)?

Stopping on the last letter of the words "lillah" (to Allah) and "al-Nabi" (the Prophet) by vocalizing them with a sukoon is correct when pausing. However, if there is no pause, it does not affect the validity of the Tashahhud, nor is it considered a grammatical error that alters the meaning, and the prayer is valid in any case.