There’s a few things here:

• Evolution does not care about our happiness. If an elite group of wealthy cannibals took over the world and created a vast breeding farm to produce trillions of humans, selecting for diverse genetics to produce the widest variety of flavours, textures, sizes, and strengths, then that would be evolutionary advantageous over our current situation. Domestic cattle have a huge evolutionary advantage over their wild cousins who struggle to survive at the margins of civilization.
• Humans evolved culture, which at its core represents the storage and transmission of information, much of which may provide survival advantages (culinary and cleanliness practices for example). Since that point, we’ve developed vast amounts of technology which facilitate and accelerate the transmission of information, allowing us to organize ourselves on larger and larger scales, some of which had enhanced our fitness (food production, medicine, improvements in shelter and sanitation) while others have reduced our fitness (contraceptives have pushed population growth below replacement levels).
• On that last point, I think it will ultimately be a temporary blip. Evolution does not care about happiness, so a large empire with no birth control and a growing but oppressed and unhappy population is evolutionarily advantageous relative to a liberal democratic society where everyone is happy but the population is shrinking.

Nature is cruel. Living things aren’t “meant” to do anything, they just exist and try to survive. Humans aren’t the only living things on the planet that change their environments. Microorganisms, plants, social insects, and beavers are other examples.

Try creating a new sourdough starter from flour and water to see what happens. It goes through a really cool progression of different stages which are each dominated by different species of bacteria, before settling on a mixture of wild yeasts and lactobacillus bacteria that are adapted to the acidic environment (which the micros themselves created)!

Similar things happen with the progression of forest ecosystems from early lichens and pioneer grasses to conifers and finally deciduous trees in a mature forest. It all seems beautiful and pleasant but there is much life and death going on all the time. Oh, and if you spend enough time living near forest with your window open then you’ll definitely hear the screaming of small animals being killed by predators.

The upstart queen can replace the main queen if she dies, yeah. Queens produce a pheromone that triggers the killing of upstart queens. In the absence of a queen, an upstart queen can survive and take over.

This makes sense. From the hive's point of view, there are at least two ways to ensure there is always a queen:

• always birth upstart-queen but kill them so long as the active queen is still alive. Once the active queen is sick or dead, an upstart queen will take over.
• Only birth an upstart-queen when the active queen is sick and about to die.

Since the latter is more risky e.g. active queen might die before birthing a new queen, the hive goes with the former strategy.

The idea with cancer being selfish comes from an idea of organisms functioning at different levels of organization. Single-celled organisms, colonial microorganisms, multicellular life, social animals, larger societies, civilizations, ecosystems, the whole planet.

I've had similar thoughts regarding the "scope" of an organism. How human individuals think of themselves as separate from other humans but the earth might think of humanity as a single organism.