Mass effect Canon

1. That is correct. Some of the novels take certain decisions for granted (e.g. IIRC Retribution states that Anderson is not the councilor) but even then, Karpyshyn said that he only sees the the novel taking lace in just one of all possible ME timelines. BW itself never established a canon and they were careful not to go into any of the trilogies plot decisions when they made Andromeda.

2. I am not sure if there is an official line on this. For me personally, I don't see MP stuff as canon as I think a lot of the stuff that is going on there is purely gameplay related and doesn't make any sense in the lore (though admittedly, even without MP, ME's lore is shaky at best ever since ME2). That said, when they added the Batarian classes to Andromeda's MP, they did spend the effort to explain why a Batarian would be in Andromeda, so I am not sure what the dev's stance on this would be exactly.

3. I don't have any links, especially since a lot of the quotes would have been in the old BW forums, which are no longer online, even to just view them.
I am sure that something can be found in the depth of the internet though. Maybe someone else will dig it up.

Drew Karpyshyn was the lead writer of ME1, co-lead writer of ME2 and author of the three main ME novels (we don't talk about the fourth one around here ). Basically, he is considered the as the guy who came up with most of the original ME universe story concepts in the first place.

Well, it means that there is no canon for the games. Whether you want to call that "alternative universes" or just "no canon" doesn't really make much of a difference as far as I see it.
Just as a side note, that line about Anderson, I talked about above is the only line in all three novels that defines a player decision from a game. (and it was a throw away line anyway, not sure why he put it in really). Otherwise, even in the novels, they are very careful not to define any player choices. For example,while Shepard is mentioned here and there, it is never said whether "the commander" is male or female.

Yep.

Yes, since the Andromeda Initiative leaves before the end of ME3, there is no official story that describes the ME3 ending or it's aftermath outside of the game itself. Therefore, there is no canon ending.

Not sure what you mean by this. But I was also talking about all the online stuff.

To quote urban dictionary: Lore (as it applies to ME lore) is "the collective history and the sum of all knowledge available about a certain fantasy or sci-fi universe."
I say ME's lore is shaky because there are a lot of retcons (changes to established fact in a later installment of the story) and inconsistencies that have been introduced into ME's background story over the 4 games.

Well, yes, I remember reading posts from BW devs, stating that there is no canon in ME.
Best I could find with a quick google search is this exchange on FextraLife (they copied over most of the old BioWare forums before they were taken offline):
fextralife.com/forums/p5598487/how-canon-applies-to-mass-effect/

Hope that answers your questions.