So I understand that The Elder Scrolls: Legends takes place in 4E 175 during the Great War, but is it canon at all?
Canon in the Elder Scrolls series is not so easily defined, especially because of the open nature of the games.
The player can, and will, take different paths and choices, that can differ a lot to the "official" storyline. This is addressed in-game and in-universe by the concept of Dragon Break, and the most prominent example is the Warp in the West (the Daggerfall storyline, basically).
According to an interview to Lawrence Schick, the Elder Scrolls loremaster at Zenimax, as reported in the forum of The Imperial Library:
All the lore in the game is delivered from the standpoint of people in Tamriel. In that way, Elder Scrolls is different from most fantasy campaign worlds, right? I mean, the typical paradigm, you know - George RR Martin with Westeros, Tolkien with Middle Earth, the familiar D&D worlds of The Forgotten Realms or the world of Greyhawk - those all have histories and backgrounds that are all laid out and they’ve all got some lore-daddy who decided everything and everything is ‘this is how it is’, so everything works within the envelope of things that are already decided.
Elder Scrolls - Tamriel - does not follow that paradigm. In Elder Scrolls, all lore is delivered not from on high by revelation, but from people who live their lives in the game, in the world of the game, and based on their beliefs. So that does two things for us: It means the lore always carries not just information about what the person is talking about, but also information about the person and their culture. Because the way the lore is delivered tells you how they believe things actually work in the world.
What this means, of course, is that people have different viewpoints - these viewpoints sometimes contradict each other, and so sometimes we have players saying “alright, this person believes that, and that person believes this other thing, but which one’s the real thing?” Well... it’s not a world like ours. In a world like ours, where you can sort of trust in science and say “well yes, people have different beliefs but I know there is an objective reality.” This is a world of myth. This is a world where reality is actually changeable, where the Divines can change not only what happens going forward, but what has happened in the past. So, you know, the idea there is an objective reality behind all these different people’s opinions is not necessarily the case in the world of Tamriel. So listen to what all these different people have to say, make up your own mind, make up your own beliefs about what happened and you’re as liable - since you’re playing in their world and you’re playing a character in their world - what you think happened is as legitimate as what that NPC thinks.
Which, to me, sounds a lot like "we know that we have an enormous world, with many different storylines, many contradictions and retcons, and many stories and concepts that directly go against each other, establishing a fixed canon is not really feasible, and we are fine with this. You decide your own canon, and every contradiction that you find is explained with 'it's magic' ".
The key concept, I think, is that the canon and the lore in TES is made to serve gameplay and to be supportive of the games, and not the other way around.
Said that, I'm not too familiar with Legends, but according to the previous principles, it can be considered canon like every other game.