Does receiving a secret recruit signet guarantee an S officer?

No the complete officer is not a guarantee. A fragment is with possibilities of more than 1 fragment and a low chance of a complete officer.

Not necessarily - given the rigged recruit spin with the normal C-B class officers, you'd most likely just earn fragments. The difference is that there are only two types of full officers to get there, and eight varieties of fragment quantities (from the same officer types). The officers to get in the secret signet recruit are:

Mr&Mrs Sam (Troop Type) S-class Officer

Reindeer (Tank Type) A-class Officer