Before I ask, keep in mind I am still playing version 1 so I will be asking this question from a v1 point of view:
Last night my brother and I blew each other’s LAST ship up simultaneously! He had Rey (ps8) with one hull remaining and i had Dalan Oberos with adaptability +1 (ps7 +1 = ps8) with one hull remaining. He had initiative and we both sent 3 regular hits through. We played it as a draw after we couldn’t find any direct answers in any forums - I suspect this is a rare occurrence. Did adaptability save me? Was this match a draw? If so, would it also have still been a draw if one of us only sent two hits through as opposed to three or would it not have mattered seeing how we only had one hull left each?

For the final results, all ships dead was never covered in the regular rules, so I would happily just call it a draw and have a good laugh!

In tournament Regulations however, they introduced Final Salvo which was intended as a way to officially resolve a draw to discourage tournament players from declaring an intentional draw instead of playing out a mirror match. A Final Salvo is used in cases where a) game ends with Mutual Destruction , b) game ends at Time and both players have destroyed the same amount of points, or c) players declare a draw. Here's the copy-paste from the tournament regulations:

"Final Salvo
If both players have the same score at the end of a game, they must fire a Final Salvo to determine the winner. To fire a Final Salvo, each player adds together the printed, unmodified primary weapon value of his or her remaining, undestroyed ships and rolls attack dice equal to that number. The player who rolls the highest total number of combined hits and critical hits wins the game. If both players roll an equal number hits and critical hits, they roll again until a winner is determined. If a game ends in mutual destruction, each player adds together the printed, unmodified primary weapon value of all ships in his or her squad and rolls attack dice equal to that number. "

And to confirm your method, dead is dead. The amount of overkill (2 extra hits vs. 1 vs. exactly enough damage) does not matter. With adaptability in play you are correct that you finish out the simultaneous fire and kill each other to achieve Mutual Destruction .

The last iteration of Tournament Regulations, FAQ, and Rules Reference for X-wing First Edition are still available on the old product page, found here:

https://www.fantasyflightgames.com/en/products/x-wing/