What is the rotational symmetry number used to calculate the rotational entropy from an ORCA frequency calculation?
Your #2 is exactly right, as far as I know, which answers your #1.

A key thing to remember is that the rotational symmetry number doesn't include reflections, so molecules in groups like $C_s$ still have $s_n = 1$ since there are no superimposable orientations achievable only with rotations.

For reference: Table II of the NIST CCCBDB introduction to thermochemistry is a nice, concise summary of the symmetry numbers for the common point groups observed in chemistry.