My understanding of the position is as follows:
(a) The B1900 is certified for single-pilot ops by the manufacturer (Raytheon). Nothing to do with commercial flights or not.
(b) The FAA has an additional requirement, relating to the number of pax in commercial ops, as set out in the FAA-approved manual (which applies to FAA-registered aircraft or ops in the USA)
(c) However, the FAA's requirements have no impact, implication or applicability to South Africa (not yet, anyway!!)
(d) The SACAA requires 2-crew in any aircraft being operated commercially in excess of 12,500lbs MTOM. Naturally, both pilots must be commercially-licensed.
(e) Under normal circumstances, the PIC in such aircraft must hold an ALTP
(f) However, where the aircraft is manufacturer-approved for single-pilot ops (NOT approved by the FAA, but by the MANUFACTURER), the SACAA permits a CLP to act as the PIC in an aircraft requiring 2-pilots.
Which is why an SA-registered B1900 may be operated commercially with 2 pilots, both of whom are CLPs.
And I don't believe any Insurer would have a problem with that.