The SQ-only RTW has nothing to do with *A rules -- or prices! That may be one reason no backtracking is allowed, and only the one transit in SIN. But, for better or worse, that's the rule: you cannot pass through SIN more than once.

There's a long thread on this subject in the KrisFlyer sub-forum. Last entry was June 13th, so it's pretty easy to find -- about 5 pages back from today's entries. Among other things, there are copies of the SQ-only RTW rules (for fares FEZSQ, CEZSQ, and YEZSQ, which are the ones you're asking about).