Re Q1, I don't think people will mind either way. Personally, I like to use the san.
Q2 - It varies from culture to culture (and over time within a culture). In the UK we tend to use surnames in formal situations. In the US it's probably the same - only the boundary between formal and informal is different. So people get the idea in the UK that speakers of AmE are uncomfortably familiar and in the US that speakers of Br E are stand-offish. So, in your case, you need to be sensitive to the culture you're in.
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