I don't think that a kitchen bin is the best place for a used diaper, but it's a reasonable guess as to where the host might want it. Unless you've specified otherwise, it seems OTT to categorize a guest putting the diaper there as rude, since it is a common choice. Similarly, the bathroom can is a reasonable guess as to where it should go. On the other hand, for a host to specify putting the diaper in the bathroom vs. the kitchen trash is very reasonable. I'm not sure that having a guest take it home or take it outside is as reasonable, particularly depending on the duration of the visit and the weather.
I can't wrap my head around not changing a child in or very near the bathroom, since I would have thought that easy access to running water to wash either the child or ones own hands was important, but I've never had to change a baby. It would seem risky to me to use a carpeted floor or bed since they are hard to clean if disaster occurs. Wouldn't a dresser or table (not the dining table) work better? Aren't most changing tables a hard surface for a reason?
Re washing the child, that's what baby wipes are for. I know a couple of people who for green reasons use wet cloths instead, but even they typically use disposable wipes when out. As PPs have discussed, in case of an absolute blowout emergency yes you'd want running water, but believe me you *know* you need this beforehand - e.g. when you realise that the child has exceeded the limits of their nappy/diaper due to evidence on their legs or clothing. Fortunately that's rare!
Re adult hand washing - obviously I usually do wash my hands after changing a nappy, but that can't be done until the child is completely dressed and safe regardless, e.g. I couldn't leave them lying on a change pad while I do it. So it doesn't make much difference whether the sink is in the room or if I pop into the bathroom wash my hands once baby is back playing on the floor or being held by somebody else. But yes, proximity does make sense. I found the hospital stay after childbirth very convenient because the sink for hand washing was in the room - so nice and close to where I was changing baby!
And in my experience change tables are almost never hard surfaces. The ones at the mall are, but ones in people's homes are usually cushioned pads. Remember the baby's head is lying on the surface and they're quite tender. When I change my toddler on a hard surface in public I usually get him to stand up for the change. With a small baby I put soft wraps/blankets down before changing.