Yes, well, QS is nasty except for those who are brilliant sight readers. It was my weakest part and I just scraped through with the minimum marks (6/15)

. There are a few of us from this forum in the "6 point club", so it's no disgrace!
Other tips...
you are allowed to play only your key chord and starting note on the piano. But you are allowed to play them as often as you like. So I played them almost constantly during my 5 mins practice. That meant that I really had it clearly in my head for the actual sing-through. This really paid off for me; I made some mistakes and didn't follow all the dynamics, but on my mark sheet I had something like "some errors but maintained a strong sense of key" (ie at least I started and finished in the right place, and I may have sung some wrong notes but they were still in tune wrong notes...).
You are allowed to relocate your note from the piano if you get lost during the actual performance. I knew that if I did this I would definitely fail (it takes me several seconds to identify a given note on the piano

) so I stood well clear. This gamble paid off for me, but in your position you may prefer otherwise - with a G5 in piano at least it's not such a foreign object for you. But at least think through what you might do beforehand so that you don't get flustered by the prospect.
Look out for changes of time - mine was mostly in 4/4 but went into 3 for a couple of bars. Also I had one tricky patch with a lot of accidentals. I only realised afterwards that it was effectively a change of key from F to E flat for one phrase. If I had spotted that sooner it would have taken a lot less time to work out all the intervals.
Whatever happens, keep going. I sang a wrong note near the beginning, and had to correct myself, but then carried on regardless. 20 years of Am-Dram "the-show-must-go-on" attitude came to my rescue there! I'm sure that this counts towards the marks. What they are looking for is a performance.
As I mentioned before, one error I made was not to mouth the words while I was working out the tune during my preparation. So at one point in the exam I started singing a word, only to have to change my pronunciation part way through (I can't remember the actual word now, but it was like starting to sing qui... as in "quiet" and finding out as you go on to the next line that it is "quizzical").
Do sing, hum, whatever helps you, during your practice. The examiners will be in the room but they won't be marking you at that point. Don't be self conscious about it. You want to pass!
Hope this helps...it's made me feel nervous again just thinking about it!