There are plenty of things we can't kill and, in fact, live on things we might use to kill them. Extremophiles that live in environments nothing else can. Bacteria that live off gamma radiation. We would have to dedicate ourselves to ridding all life on purpose to kill everything. We would have to live long enough to be the last things to kill if that was the goal.
Eh I doubt it. Every single nuke ever built combined still doesn't come close to the power of the Chicxulub asteroid (the one that killed the dinosaurs) and even that impact didn't come close to eliminating all life on Earth. Unless someone accidentally compresses a mountain into an artifical black hole or something there probably is no way to wipe out all life on Earth.
75% of all species, not all life. Larger species and photosynthesizers were more heavily affected, while smaller species, scavengers, and deep sea life were less affected.
And I'm not a biologist, but I'm pretty sure even 75% of all life, not species, still wouldn't be close to completely ending life on Earth, cause in the end as long as some microbes survived around a hydrothermal vent somewhere total extinction would be avoided.
I still think that "lifeless rock" does not specify how lifeless - theoretically extinct or just lifeless enough to make human life either extinct or just miserable. I took it as the latter, and in that case even lesser cases than 75% of all species would suffice.
The first case, the theoretical and non-human focused pov is quite another thing. Like you said, there's so many opportunities and adaptations for life to seap through the combs of doom :)
Even if humans manage to kill off most life on Earth it will continue to exist, propagate, and become more complex. Again we’re talking about billions of years. There have been huge shifts in climate and mass extinctions many times before and yet here we are.
So I took it to mean all life on Earth being dead. As long as one microbe survives to reproduce and start evolving it doesn't count.