Is it necessary in any case to carry an additional crew member on the balloon?

Oh, yeah, yeah. The requirement BOP.ADD.410 states that if more than 19 passengers are transported on the balloon, an additional crew member must be part of the crew.

The acceptable means of compliance AMC1 BOP.ADD.410 sets the initial and recurrent training requirements, as well as recent experience, to be met by the additional crew member.