I am confused- You say your alternator died suddenly and it was confirmed by your shop but you don’t say why you and your shop concluded your alternator was bad. You then tell us the bad alternator tested good at the auto parts store and relate in detail what your voltage reading are with your new alternator but don’t tell us what they were with the old one on the car. With the information given I have no reason to think that either one of you alternators were bad.
With a 14 tear old car that has had the engine and transmission changed i would suspect a crushed wire or bad ground someplace. With one person reading the voltmeter , have another person push and pull on all the wiring harnesses.