it takes about a week to dry the buds, so it sounds like you'll be right on time, when you get back they'll be ready to toss into mason jars and cure, no need for the freezer
but what are you doing fimming so late in flower? you should fim during veg well before you even kick it in to flower...
epsom salt is probably your cheapest option, it's only $2 or so for a huge bag of the stuff, and it can be used along with rubbing alcohol for cleaning out your bong =)
you can flower your plant whenever you like, but remember it's going to double (or maybe triple) in size from the time you set it to flower
seedling to harvest in 2 weeks is definitely way off, it's 2 months for flowering alone, plus however long you veg it for
a layer of sand on top of the soil will not allow the larvae to escape, along with a hotshots no pest strip to kill off anything currently flying around