Old Jewry still exists as a street. It may jar, but it is part of the history of London, and evidence of an evolving city.

I think Mrs Mason, as a good nurse, had a good idea of what Jack was really like, based on long experience and not being blinded by family attachments or long acquaintance. If Jack was aware of this then he would not have been happy to be with someone he could not hoodwink.

I rather imagine that any ship Jack took was doomed to sink, since that was what normally happened to people in ACD canon.