Are my previous prayers —in which I addressed the Prophet, peace and blessings be upon him, with the vocative particle “ya” and intended to convey greetings to him, not supplicate for him— invalid, given my ignorance of the ruling? And must I repeat them?

Adding the vocative particle 'ya' before (O Prophet) in the tashahhud is not prescribed because it has not been reported. However, it does not invalidate the prayer because "ayyuhā al-Nabī" (O Prophet) is a vocative noun from which the particle 'ya' has been omitted for lightness and frequent usage. For the worshiper to envision, when saying: (Peace be upon you, O Prophet, and the mercy of Allah and His blessings), that he is greeting the Prophet, peace and blessings be upon him, there is nothing wrong with it, and it does not invalidate the prayer; because Allah has commanded us to send prayers and peace upon him, and the Prophet returns the greeting to whoever greets him. Sheikh Al-Islam Ibn Taymiyyah mentioned that the saying (Peace be upon you, O Prophet) is a call intended to bring the addressed one to mind in the heart.