To answer your question mik5, we would need more information, the most pertanent being at which latitude you are residing? If you are as far North as Rabbeinu Tam & his contemporaries resided, then there may be some serious obligation to await 72 minutes. Assuming you live somewhat closer to the equator, and if you reside in the USA or/& Israel this is the case, clearly 72 minutes is way longer than the time it takes to get dark & have 3 stars emerge, therefore some fabricated Chumro cannot override any Chiyuv; This includes absolutely not keeping leather shoes on your feet past regular Zeman on Tischo b’Ov nite, not delaying Channukah candle lighting beyond 1/2 hour from real Zeman, not delaying Seder nite commencement when kids may fall asleep & you may miss Afikomen by Chatzos.
As for your specific questions, you absolutely must participate in the Minjan for Maariv, and you were absolutely Yotzei Havdolo at your Shoule.