If it helps, think of it like this… “What species is likely to have caused that airplane to be there?” in the same way as “What species is likely to have left that footprint in the sand?”. In the case of the former, it’s a pretty good bet it’s Homo sapiens (the pilot!)
A1 No. “Abiotic phenomena” would be things like coloured rocks where the colour is not due to a living organism. Or a cloud shaped like an elephant!
A2 Yes. In the guide it lists both humans and “abiotic phenomena” as observations that are not encouraged, but acceptable in small numbers. As an example, I might make 100 observations of animals I saw on safari, but then I see a cloud that looks like an elephant, and I find it funny that I don’t have an observation of an elephant while I was on safari, so I make one of that cloud. 1 in 100 observations makes it sort of ok, but not to be encouraged to do so! Some people take the humour too far and make an ID of elephant, but the appropriate ID is Homo sapiens, because that is the only legitimate species involved in the observation (remember, every observation has a human involved, by default… “the observer”)
A3 There is no abiotic category to put observations of “buildings, streets, vehicles” to, so they are always put as Homo sapiens because of the involvement of the observer (see A2). If there is a hawk sitting on top of the vehicle, then it can be put as the hawk for ID, but only if the observer doesn’t specifiy the subject of the observation to be the vehicle! The observers’ intention for subject is always honoured where possible.