You can miss it out. Go direct from Ayas hut to Gnifetti. Just ski down from Castor as normal then hang a left once you meet the track going up towards the Naso. We did this some years ago and Mrs J even found time to do Pyramide Vincent (while I sat in the sun... )
Have you been too skimpy with your route suggestion? You are certainly correct to warn about not turning off the descent too early as the route doesn't follow the oddly marked way on the map: you need to continue to the Felikjoch and once descendibg the SSE ridge from Felikhorn parties need to be careful of the West face as this can be heavily loaded. The way across fom the col between the Felikgletscher & Lysgletscher should be well furrowed but in poor vis good nav is important whether you are missing out or coming from Quintino Sella.
That said and in an ideal scenario, I would prefer to stay at QS as the Gnifetti is so accessible as it is frequently horribly crowded & whilst the staff are unquestionably hospitable and helpful, some clients who may only be tackling the 'easier' ticks and have come up from the ski grounds to the S, can be very loud and uncaring towards fellow travellers - in Summer, dinner time is a mayhem of folk scrambling to be at the front of a 120+ person queue!
James, I popped into the Quintino Sella las week on the way down from a ski tour. The winter hut is very comfortable - gas cooker, wood burner, large dry dormitory with plenty of blankets. So carry a little bit of pasta and stay there anyway!