Sierra259's board-to-board with Subtank

By now, that discussion is not reliable anymore (2009 is old). It was back when Bungie was strong as ever and 343i was still unable to figure out how to manage the franchise properly :P

The visuals of Halo Wars was not properly done; as the discussion shows, Ensemble artists had no idea of the franchise's strict adherence to canon and use that innocent knowledge to liberalise the visuals to an extent. The notion that "Bungie supervised Halo Wars development" is false; there is little Bungie participation in the game's development and that the notion was made simply to reaffirm to the fanbase to calm down and still buy the game.

As to answering your question,

1. There is nothing legal about canon. Legal has nothing to do with canon. Canon is a fictional concept introduced by the creator (Bungie) and enforced by the fans (Halo community) to make sure the developer of the franchise (Bungie, Microsoft, etc) is consistent with previous title. Halo Wars is in similar canon position as other Halo titles, as long as it does not contradict established foundations of the canon Halo universe. Visuals can differ from time to time, but consistency is key and without it, there's no point of having a canon policy (the problem with Legends; 343i took the opportunity to claim that all visuals seen is canon to avoid the throwing canon out of the franchise... but they created a mess in doing so).
2. That is the conclusion made by Specops306. I think you may have read his comments and see that that is now how Halopedia (and hopefully the entire Halo community) sees it.
3. Bungie earns a "creator's right", that is their contributions will not be thrown out of the window just because they are no longer working on the franchise. Anything released by them that has a direct influence on the franchise will still hold some weight, but that would only fall within the period they were actively developing the Halo franchise.