The problem with answering this question is that what Americans think now is a violation of the 20 year rule (and not a historical question). But in terms of what have Americans thought, the generalization is to say that most Americans have traditionally thought that the bombings ended the war and prevented even greater slaughter. This is because they have been told that since 1945. A smaller subset has always believed they were probably unnecessary and could have, and maybe should have, been avoided. The number in each camp has varied over time, and in recent years it is split largely along age lines (older Americans feel the former, younger feel the latter), though those age lines are also political lines (older people tend to be conservatives, younger tend to be more liberals).

As for your question about "guinea pigs," I doubt many Americans know anything about the attempts to study the hibakusha nor the "no treatment" policy of the Atomic Bomb Casualty Commission. We certainly have no polling data on that. So it is not an answerable question. It should be noted that the hibakusha could of course receive medical treatment. It is just that the ABCC did not see their role as one of treatment, and interpreted treatment as a form of "apology" or "atonement" for the bombings (which they refused to give and did not see as their role to give), and so provided medical treatment as a form of compensation for cooperation with their study. One can argue whether that is wrong or right (it did create resentment among the Japanese, but the ABCC's position is not totally impossible to understand), but it is different than, say, the Tuskegee experiment, where treatment was actively denied for the purpose of research. On the ABCC's "no treatment" policy (and ABCC's history in general) see M. Susan Lindee, Suffering made real. But again, I don't think your average American ever knew about the ABCC, much less their policies.