I discussed the problem with the fluent expert at my end and following are the suggestions:
Outlet: It depends on what value is available with you to be provided as output. In case mass flow rate is known, it is better to use. You already know that using mass flow rate allows non uniform distribution of flow at outlets.
Inlet: Using inlet BC with pressure is best as this is a suction problem and for such problems it is better to use a zero value of Gauge Total pressure and not just the Gauge Static Pressure.
Though for this problem, using opening or inlet will both give results in close proximity but using inlet with total pressure is more accurate as per the guidelines.
I hope this answers your query. Please let me know if there is any follow up query.