Just thought I'd report on my experience.
Didn't buy subscription, got Economy+ on EWR-TLV segment at check-in (which was done a day early at a United Club in DCA) so we can all be seated together.
On TLV-EWR Economy+ seats were very scarce, a few days before the return there was no E+ option to seat us all together, but 5 adjacent seats did open up in Economy. Rather than taking the 5 adjacent seats, I took 4 of those leaving one middle seat empty, and another isle seat elsewhere on the plane. After boarding I switched my isle seat which was further towards the front, with the person seated in the isle seat right behind my kids. Worked out very well.
BTW, on this itinerary SQ credited 100% of miles flown, AV credited 50% of miles flown, UA credited based on fare which ends up being approximately 20% of miles flown, but did credit 100% of miles flown towards PQM.