It's not an issue of SQ screwing/scamming passengers. It's a quite simple matter of SQ not offering roundtrip mixed Saver-Advantage awards. Unfortunately, the agent didn't describe the limitation properly. As THR mentions, the rule of thumb with SQ is to always book 2 one way awards to allow for flexibility. SQ grants this one way flexibility with no penalty/higher pricing. Had the OP done that this change would be quite simple.

And welcome CX8888...16 years from joining FT to first post!