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.
It was not enough just to kill Mar-Vell, because even if he died, his bosses and the whole Cancerverse would still exist and would still threaten to invade our universe. As was explicitly explained in the comic, in order to make the beings in the Cancerverse mortal again, Mar-Vell, the avatar of Life, needed to be killed in the same ritual place where had killed the avatar of Death. That's why Death waited until the moment Mar-Vell was there to get him.
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 Many-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.
When Drax puts on his old costume, he doesn't seem to have any of the red tattoos that were part of his new character design. What happened to them?
He's wearing a shirt, so the tattoos on his chest were presumably still there, just not visible. As for the ones beneath his eyes, those weren't present when he was reborn during his mini-series before Annihilation and only appeared later, so presumably they were makeľup and not actual tattoos. (Gamora's similar eye marks are definitely makeľup, as they switch between red and yellow depending on her outfit, and are sometimes not present at all.)
If conquering Death is as easy as killing her avatar in a ritual, why does she even choose to have such an avatar? Doesn't that make her incredibly vulnerable?
Clearly Thanos isn't unkillable, since he was, you know, killed. And as for Did You Just Romance Cthulhu?... In the 616 universe at least Thanos' love affair with Death is fairly one-sided, Death doesn't really seem to respond to Thanos' emotions. We don't know for sure if it was the same way in the Cancerverse, but since that universe and the 616 universe apparently were similar up until the point where Captain Marvel was dying, there's no reason to think their relationship was any different.
If I recall correctly, during the Thanos mini that served as a prelude to the Annhilation event, Death does admit that she holds feelings for Thanos (to the extent Death can love somebody, at least), but the problem was that Thanos never knew how to court her proper. She was never interested in those cosmic demonstrations of power that he was so fond of.
Well, while having a mortal Avatar does make Death vulnerable, it also allows her to influence the mortal realm. Death can't actually do much without a mortal Avatar to do things for her, so if she wants to do anything other than watch and hope for the best, she needs to accept that vulnerability. Again, Thanos is probably the best choice.