I never stated I had went through an entire minefield. I stated my ships were attacking the minefield.
In determing the location of the mines, I used the same theory you stated at Mon Calamari - that they were gathered around the yards.
Therefor, it was a simple task for the 1227s to avoid the mines and get into the atmosphere. You will also note only a handful of generators were disabled (not, not destroyed, disabled), those being the ones that protected the planet immediatly under the Imperial fleet.
If you had responded to the thread, the damages would not have been as severe. However, your failure to respond is taken as a sign that you don't care about the planet, and so total damages have been stepped up (though only slightly).
You are still entirely able to defend the world, its static defences have merely suffered some loss.