Master and Commander Achievement & Extended Cut Error

At what point did you download EC?

Beginning "Priority: Cerberus Headquarters" locks down your EMS, so jumping in after that mission might cause an issue (I haven't tested this, so it's a bit of a guess).

"There seems to be other issues with the mix of Leviathan DLC and Extended Cut DLC that do not allow the extended cut scenes to play after completing the Leviathan DLC."

The only thing I've experienced are the additional flash forwards (when you ask for more information about your options) not playing, which end up giving Shepard an odd little head tick. I read somewhere on BSN that this doesn't happen when you start a new game with EC and Leviathan installed, but I haven't tried it yet.

At no point did I ever start a new campaign. I continued my original play though very slowly from when ME3 first released.

I downloaded EC when it was first available and continued playing the game when time permitted.

I then downloaded Leviathan when it was available and completed the Leviathan DLC before continuing the rest of the campaign. I then continued the game (which was about the point of Priority: Citadel III as I had just completed Priority: Rannoch before starting Leviathan) through to completion.

Upon choosing my ending (Control), I sat back and watched the cut scene (without ever watching/reading any spoilers) not knowing that I wasn't watching the extended ending. I then went to my PC to review the un-extended ending and see what the difference was. To my surprise there was none. It was the same ending. The EC never kicked in. I then went through my achievements and realized that the Master and Commander Achievement did not unlock either. Then I began going through the menus to see if there was an option to rewatch the endings... which there isn't. I realized I'd have to try again... so I started from a save point just before Priority: Cerberus HQ and the achievement still didn't unlock.

I'm not sure if the Master and Commander achievement should unlock upon starting Priority: Cerberus HQ or if it doesn't unlock until the end of the game, but neither happened for me.

So you didn't get monologue from Shepard at the end?

Negative.

I only got the original ending. After looking up all 3 extended endings on youtube, it's safe to say that mine was no where close to what was those were.

Is it safe to assume that I will need to replay the entire campaign to get the achievement at this point?

Can someone verify whether it is calculated by score or scanning of the galaxies?

Someone on the Mass Effect forums mentioned that they uninstalled the Leviathan DLC and it fixed the ending glitch. It may be possible that doing this and finishing the game again could also fix the achievement error. I will try it, but I won't get a chance to finish it for a few days.