Usually the type certificate gives answers to some of these questions but in the case of the S76 it doesn't help too much.
All variants from the A to the D are covered by the same type certificate.
The Arriel S76A variants only get a mention in the notes whereas the three different engine configurations for the S76C 1S1. 2S1, 2S2 are covered individually.
The S76A Arriel versions are covered by STC.
I think the designations +, ++ are Sikorsky marketing department devices as they do not seem to appear in FAA paperwork.