My guess would be that 身, in this case, means "one's position" or something like this, but how does that really work? For instance, if I were to say アメリカ人の身で in a context where an American wouldn't be appropriate to talk about something, would that work as "Even though he's American..." or "Being American.." (in a negative way)? It reminds me of 身の程知らず.
Additionally, I'd like to know what もの expresses here. Is it something to express a feeling of surprise?