What is the 'right' way for a 3-Phase Watt Meter to determine import-export Watt-hours?
You are correct, their are two ways 3-Phase Meters can determine Import/Export Wh totals. I am a Electrical Engineer and have programmed EDMI smart power meters for customers in NZ who provide reconciled Import/Export kWh data to retailers for billing purposes.

Option A is sometimes referred to as Net Metering and Option B, Gross Metering.

Net metering is only really useful if you have a deal with your Power Company where you can buy energy off them for the same rate that they will buy it off you. This doesn't happen very often, as in practice it costs them more to generate and distribute the energy than it will cost you. It is also illegal in some countries to take the net totals in the meter. In my opinion you are better keeping the gross totals in the meter and then performing the net calculations downstream in the software reading the meters i.e. Option B.