Why, to get Death close enough to Mar Vell, did Thanos have to be killed in that ritual place. Wouldn't Death have been summoned the moment Mar Vell killed anyone in the regular universe?
It would appear that causing death is not enough- Mar Vell had to get close to Thanos, and in that place where Death was first killed, try to kill her again- Bringing himself extremely close to Death once again, but this time, with Death fore-warned about what was going to happen, so she could strike first. As Thanos says, he cheated death, and nobody does that twice- So yeah.
Did Thanos not exist in the Cancerverse ever? How did that happen?
Presumably, Thanos did exist, and was the avatar of Death. And was killed by Mar Vell, as a part of the Necroscopy, to kill death.
How did Cancerverse Captain Marvel kill Death? Has Captain Marvel always been that powerful?
The Many-Angled Ones had a greater influence on the proto-Cancerverse, and we're able to influence Mar-Vell and the Avengers into performing a series of dark rituals that culminated in the death of...Death.
Why did the Many-angled ones need Death to invade?
Presumably, the Many-angled ones can't grow infinitely in a realm where they can die.
What happened to anyone who gets obliterated in the Cancerverse if they cannot die?
The Revengers were shown to regenerate, unless Thanos offed them.
How did the Scarlet Witch hold off the Man-angled ones when the other magic users couldn't? Weren't all the Avengers turned at once?
It seems to be less a question of power, and more a question of choice. Keep in mind that all of this happened because they couldn't accept someone they cared for dying, so they made a deal with the devil. Scarlet Witch didn't want to make that deal, but pretended to go along with it.