The latest evidence is that for most people the whole gait analysis thing is a bit rubbish. What matters above everything else is comfort. If your current shoes are comfortable and not causing any problems then buy some more of them.... or the current version. You are taking a bit of a risk buying a different version without trying them on but I buy all my shoes online and mostly they've been fine.
In your position I'd get a pair of Lunarglide 6 or 7 from Sportsshoes.com and see how they feel.
Your nail gets damaged because it's hitting something... repeatedly. It could just be rubbing on your sock but it's more likely to be hitting the end or the top of your shoe. You need different shoes.
Lots of people choose a bigger shoe size for their running shoes. I've never heard of anyone choosing a smaller shoe so that is very likely to be your problem. If you aren't happy with your current shoes your best option is to go to a running shop and try on lots of shoes until you find a pair that are comfortable and where your toe doesn't touch the end.