]I wonder why JA did not tell us how Lizzy would call him! I believe there have been lots of discussions in the P&P board about this, but I think she will call him 'Darcy'. My theory is that- Jane speaks of Mr. Bingley as 'Bingley'...

But when they said 'Darcy' or 'Bingley', they were speaking of them and not to them. When they speak to them I believe they will say 'Mr Darcy' and 'Mr Bingley', or they might call them by their Christian names.