But instead, they co-operate. They don't necessarily agree on everything, but say Elizabeth doesn't back the Lords in the 1560s against Mary - since, according to Leanda de Lisle, Elizabeth was more in favour of the Stuarts absolutism than the suspected "parliamentary co-operation" of the Greys - and Mary doesn't actively focus on pissing Elizabeth off.
Darnley would probably be a bone of contention between the two, so let's say he either marries elsewhere, or drops dead before he gets to Scotland. Sure, his younger brother is available, but Charles Stewart is even younger than Mary. I'm not sure who Mary would marry in place of Darnley, but I was genuinely wondering if this is at all possible?