Sometimes the QF lounge at SIN does get full, and invokes the capacity restriction; but this only happens at their hub time (lots of BA/QF flights converge on SIN). Note that QF built and runs the lounge (BA pays QF to use it, but it is not a joint BA/QF lounge).

The J section of the Pier is substantially the same as the J section of the Wing (just underground); it is only the F sections that have different amenities. Good exerecise walking between them (and you get to see all the planes, plus some of the shops along the way are interesting).