Not much to add as Tracy has given sound advice as usual!
I would stress the coed/single gender issue is extremely important to my wife and I. However, it's an ideological issue for us, we have no medical or religious grounds with which to make a formal case.
I agree this is unlikely to be a strong argument unless you can establish why it is so important for your child.
However, this represents a slight weird artefact in the system where a child who initially fails the kent test is potentially better served by the system than one who passes.
The parents of the child with the borderline fail have two
hurdles to overcome: first, non-qualification, second: oversubscription. I agree that, if they get over the first hurdle, they might be better placed than you with regard to oversubscription, but I don't see that as an anomaly. In my view a child who is deemed qualified at appeal deserves to be treated the same as someone who qualified via the 11+. The anomaly is arguably that the 'wrong' decision was made in the 11+, but because of the lateness of the decision, the child may well be at a disadvantage as most grammar schools might now be oversubscribed.
The focus at oversubscription should be on the school being appealed for, not on the school allocated or other possibilities. However, if the panel is minded to allow more appeals than it feels the school can cope with, it must then compare all those cases and decide which ones to allow up to the point where the prejudice to the school would become too great. I accept that, in this situation, the panel's sympathies might be with the parent who doesn't have an alternative grammar school or an an excellent comprehensive already on offer, or who hasn't got a good independent school lined up - but the decision still depends on the strength of the case as a whole, taking into account all the reasons put forward by the parent for wanting a place.