What is the ruling on someone who said "samiʿa" (heard) instead of "al-salamu ʿalaykum" (peace be upon you) in the final tashahhud, then remembered and said the taslim without performing the prostration of forgetfulness (sujud al-sahw), believing that it was an additional verbal utterance for which prostration is recommended, and then doubt occurred to him? And is the prostration of forgetfulness valid for someone who said "Allahu" (God) only instead of "Samiʿa Allahu liman hamidah" (Allah hears those who praise Him) in the Maghrib prayer?

What you did by inadvertently adding the letter 'seen' is considered a verbal addition. There is a difference of opinion among scholars regarding the legality of performing prostration of forgetfulness (sujud (definition) al-sahw) for it. According to the view that it is legitimate, it is only a recommended act, and there is no sin in omitting it, nor does the prayer become invalidated by it. As for your performing prostration of forgetfulness due to uttering a prescribed remembrance in an improper place, you are correct, and your prayer is valid.