What is the ruling on saying "Amen" after the last supplication in Surat Al-Baqarah, and saying "Bala" (Yes/Indeed) after Surat At-Tin in prayer, and is there any Shar'i (Islamic legal) evidence for that?

There is no impediment to saying "Amen" at the end of reciting Surah Al-Baqarah, whether in prayer or outside of it, and the proof for this is the action of Mu'adh (may Allah be pleased with him). As for saying "Bala" (Yes, indeed) after reciting Surah At-Tin, it is a Sunnah.