I'm not sure I follow your model of IP networking, but the problem boils down this: In live networks, you don't always know which device presents your last-hop

I'm not sure I follow your model of IP networking, but the problem boils down this: In live networks, you don't always know which device presents your last-hop routable address to the public network, so it is not clear who can authoritatively answer the whatismyip question. You can get an instantaneously-correct answer (i.e. correct in the instant, not necessarily quickly-answered) from an empirical test, but you need to choose an known (and trusted) entity on the public network to query. Querying multiple different entities might very well get you multiple different answers, depending on how your packets are routed and manipulated before they reach the public network. And in all cases, there is no guarantee that your public address will be stable over time or location.