I'm somewhat impressed by your rational approach, so I'll try and point a couple mistakes you make in your reiteration of the codex. I myself - as I already stated in the post above - am quite sure that IF Red Crossing has indeed been massacred, it happened after the events described in the codex, likely when the all-out war started. Though I don't really believe that too. In my opinion the most probable chain of event is: a brief small-scale fight between the elves and people of Red Crossing occured, there were casualties and as the word of incident spread, fear and distrust progressively added more and more gory details to it until it became a massacre of the town.
But that is just my interpretation, other possibilities exist: like Red Crossing getting burned down during the war or even being sacrificed for propaganda. There is also possibility that the elves mourned for their fallen comrade and then proceeded to slaughter the village. But I find it highly unlikely, seeing the general tone found in his memorial, where pretty much all the blame for Elandrin's and his lover's death is put on Siona...
The problem with that interpretation is that the residents of Red Crossing would have noticed that they were never massacred, even if no one else did. The town still exists, presumably not with descendants of it’s original residents, but if there were survivors I think that the truth would have gotten out. The people would have their own records. Their grandbabies would wonder how it was that grandma lived in Red Crossing her whole life when she was born in 2:02 and so on.
I do agree that it's possible that Red Crossing wasn't properly massacred until later.
So, with that out of the way, let me correct you on a couple things concerning this particular codex entry and its context. Not all my corrections are entirely pro-elven, though most are: I'm afraid that there is no such thing as objectivity and you do exhibit a noticeable anti-elven bias. Still, I like your approach, I wish more people would behave like you. Regardless of what they think about elves 
Now, I would be careful with this classification. There are two issues - the "contemporary" account is, obviously, much closer to the times where the events took place, but it's not as close to them as it might seem. The tomb holds memorials of multiple other Emerald Knights, who obviously died during the war and not in a single battle either, after all. This is an important aspect I'll get back to.
This can also mean that the memorial itself is - even unconsciously - a manipulation, since if any amount of time passed between the events and writing them down... well. Time takes its toll on memory. And then there is the fact that it was written to preserve the truth that might otherwise be corrupted... The urge to include at least a little bit of interpretation rather than raw facts would likely be strong. How exactly and to what extent did this influence this account? Assuming it's 100% accurate is a big leap of faith. Even if it is a memorial.
For all we know, it could be - in extreme case - some BS written with the sole purpose to whitewashing Elandrin and blaming the whole incident on Siona 
I’ve never claimed that it was the definitive account. But the war only lasted about a decade, his memorial may not have been written within days of his death, but it wasn’t written centuries later. There is a relatively short window of time during which the elves would be in a position to build such a monument and the people who built it would either have been there or known the people who were.
Siona also gets her own tributes. Her epitaph doesn’t read “That trigger happy cow who started it all”. She’s remembered as a hero of the war, a great defender, while Elandrin has his memorial hidden away. His death is the one that shames them.
While you're mostly right, elves obviously felt threatened by Chantry's attempts at... what's the Chantry-equivalent word for Christianization? Either way, turning an elf away from elven tradition in that situation doesn't really seem to be as harmless as you portrait it. Given the apparent role of Emerald Knights - defending Dales from contamination by human influence - turning to the Chantry and converting to Maker does seem like a serious betrayal that is likely to escalate... I guess Chantry-elven relations were religiously paranoid on both ends.
I never said her fears were harmless. I suggested they were unfounded and irrational. She is making assumptions. She is assuming that he intends to convert. She is assuming that if he converts he will betray them. Spill their secrets. Lead the humans against them. These assumptions seem to be based entirely on his body language when she sees him with a human woman. Whether she would be justified in her fears if she had been correct is another matter entirely.
You generally notice ambiguity in the codex, but missed it here. In fact, the only casualty mentioned so far is that one girl. Humans were no match - but could've been driven away (especially if they weren't appropriately armed). The codex doesn't really mention killing a single one of them. While I do think there probably were casualties, codex doesn't mention them not only on the elven side.
You are correct that the villagers weren’t explicitly killed. But it is very much implied. The only realistic alternate explanation is that the knights let them run away, giving them the opportunity to properly arm themselves and get reinforcements, then waited by the bodies of the fallen/incapacitated until more humans arrived. In that scenario the elves are acting beyond stupid. Now, I am not entirely adverse to that interpretation, but it’s one that most people who defend the elves usually frown on. 
The events at Red Crossing led ultimately to the full-scale war - they definitely were not the end. And, as I mentioned earlier, the source material likely was created quite a bit after Elandrin's death, during the war, long after the events escalated out of proportion. Basically, while an interpretation "it was not the end of events Red Crossing" is possible, I find it more likely that what this refers to is what came later and was, at least partially, related to what was probably the first "elven attack on human settlement" - regardless of the actual death toll of said attack, regardless of how intentional the attack was. Elves fought humans at the outskirts of their town/village - that is the one unambiguous part. And it definitely could be considered a step up in hostilities from occasional skirmishes with bandits, chasing away missionaries or a couple hunters or traders who ventured into the woods never to return.
As a final note, there is one important problem with the classic "many accounts of massacre" argument - they're not really many accounts. It's one account, the Chantry history that became common knowledge. Much like every Dalish Keeper recounting old tales doesn't really count as seperate source, human scholars tend to fall back on the official history rather than their own findings. Perhaps the closest we have to an independent human source would be this song http://dragonage.wik...in_Red_Crossing
that appeared who-knows-when and changed who-knows-how... and hardly describes anything, even if it really is originally from correct place and period (and created by someone with any amount of first- or even second-hand knowledge).
My point, which still stands, is that the Death of Elandrin does not contradict the Chantry account, despite some people claiming otherwise. They can exist side by side, so no one can claim the one they don’t like must be wrong.
It’s possible that the events described are the massacre, with the details later exaggerated, since at least some must survive to tend to the bodies. It’s possible the real massacre happened later. It doesn’t say definitively either way.