Just to get access to the SQ First class lounge (not TPR) requires you to have Solitaire PPS status. Gaining Solitaire PPS requires a spend of SGD250,000 (250K) in first or business class on SQ or MI within 5 consecutive years. To maintain Solitaire you have to spend SGD50,000 per year in first or business class. So it's not an easy benchmark.
Would there be pax without status in F - absolutely. Not least on the basis that trying to get Solitaire is not that easy. Krisflyer miles on the otherhand are fairly easy to come by and so are first and business class awards. And the awards are cheap by comparison to other airlines. Mixed class awards are also common, simply because SQ doesn't offer first on many intra-Asia flights. Equally stopovers are promoted. So the combination of mixed class and stopover, with no status, would be common.
SQ looks after their F pax well. The arrivals benefit for TPR is unpublished but is consistently applied. No exceptions. So staff are well aware of the rules - even the 'unofficial' ones. It's therefore unlikely they've been incorrectly applying the rules all this time. But stranger things have happened so maybe CE's enquiry will prompt a correction, if this is the case.
CE is a very frequent traveller and well known to the staff, I believe his entry to the lounge was on the basis that lounge staff thought he was a Solitaire. (I can't recall if it was the F lounge or TPR though? I thought the former?)