As I said before — the constitutions (and, by extension, constitutional law) in the Commonwealth Realm countries are quite different from that of the U.S.A.
Your premise (that she could “veto”) is incorrect. You could visit a law library in Canada for the reasons why — it takes a lot of explaining. The customs and practices (and precedents) of every nation in the Realm, going back hundreds of years, have to be taken into account. Bottom line — if the Queen had withheld assent from this bill; it would likely be the last official act of any British Monarch.
And yes, “By the Grace of God of the United Kingdom, Canada and Her other Realms and Territories Queen, Head of the Commonwealth, Defender of the Faith” Queen Elizabeth is Canada’s Queen and Head of State.
Well I wouldn’t want the office around rather than having one giving ceremonial nods to something that’s about as just plain wrong as anything can be... ignore the sodomy issues for now, this legislates a nonsense definition of something that dates from time immemorial. When a society DID support homo pairings, it never stole the “marriage” terminology, till the modern craze.
But of course I can only opine, just as you can opine about what we brash Americans say.