AFAIK yes, users from instance B can downvote the post, but the downvotes won't federate to instance A, so only users on B and C will be able to see them.
Those are for special occasions, like when you're doing electrical work in someone's house who you don't like much and feel like splicing 10 short wires together instead of using a long one.
How do you use a keyboard for any significant amount of time without noticing there are two shift keys? (No offense intended, that's just kinda crazy to me).
Of course, public transit is far better. Unfortunately it's often easier (especially in the US) to convince people to buy a different kind of car than it is to convince them to use public transit.
Even when powered by fossil fuel power plants, electric vehicles are still usually better in terms of CO2 emissions. Power plants benefit from scale, so they're a lot more efficient than the tiny combustion engines in cars. Regenerative braking also saves a ton of energy when driving in the city.
Not sure what you're doing to other people, but I don't think I've ingested macroscopic quantities of anyone else's skin cells since I was a young child who was a little too fond of biting. If you count individual cells it's probably technically true that I inhale some occasionally, but that's a bit misleading. I'm still constantly practicing self-cannibalism though!
Sounds very counterintuitive, you got a source for that?