Hey, Kannabis-

If your leaves are curling inward & upward, it IS likely to be an Mg deficiency. A quick fix is to water your plants with 1 tablespoon of epsom salts added per gallon of water. You should see an improvement within a couple of days. Since you're already into the flowering stage, be careful not to overdo it, as the salt accumulation will take longer to flush out of your plants at harvest. A magnesium deficiency won't kill them in their last few weeks, but make sure they have enough nutrients to make it through their final 2 weeks of flowering (during which you should be leaching, or flushing, all the crap out of the soil with plain water).

Don't worry about which plants start flowering first. When I first started out, I was convinced that male flowers always show first, but have grown strains where that is clearly not the case. Just pray they're all girls!