Does the Maghrib prayer become invalid by repeating the recitation of Al-Fatiha after reciting half of it silently then aloud, or by being uncertain about the exact position of the worshipper (slightly behind the Imam or beside him)?

Reciting Al-Fatiha aloud again after having recited it silently before bowing for ruku' is the correct approach. In this case, some scholars recommend performing the prostration of forgetfulness (sujud (definition) al-sahw) after the taslim (definition), though omitting it does not invalidate the prayer. As for doubting the worshiper's (ma'mum) position relative to the imam, this does not invalidate the prayer and has no effect on its validity.