Except for the fact statements they made said later make their own ending feel more incorrect than some of the modded ones. So yes, the ending is "the ending" And someone fixing the scenes to better reflect "the ending" Would be just as valid as going back and making a mod for a older game in a series to reflect changes to lore from sequels.
Maybe they intentionally made the original ending janky so people would talk about it until the end of time like this, it's actually quite brilliant if so.
See, this is the issue that a lot of people don't understand -- it doesn't matter if the official ending feels "incorrect." Yes, there are good and bad versions of each ending, but the only legitimate or "correct" endings are the ones that were included in the original game and modified version that came with later DLC releases. No how strongly the author feels, fanfiction will never be considered canonical in any universe. No matter how valid your reasoning may be or how popular your alternative is, it is not legitimate or canon unless the people with the rights to the franchise say it is. You can disagree with that idea if you wish, but Intellectual Properties just don't work like that.
That being said, it would be one of the biggest marketing ploys ever if what you suggested was true. Every time there's a discussion of fan satisfaction or fans demanding a change in a game, Mass Effect 3 is mentioned. Truly brilliant if true. Yet, much like legitimate endings, it's just a wild, conspiracy theory unless somebody from Bioware confirms it. Although, there's nothing wrong with donning a tinfoil hat every now and then.