Yeah freezing the wort is ok but the yeast will explode unless you treat it with glycerin, just make some slants if you want to keep it long term.
As far as thawing out the frozen wort, just put the bag in the sink. They take longer to freeze due to lower freezing point, and thaw faster. What Ive found is that when you start to thaw it, the sugar melts fast and the water stays frozen longer, usually with the break material.
What I do is take the ice chunks out of the thawed malt sugar, and just dillute to the appropriate grav. for your starter application. Way cheaper than DME (considering it's mostly free if you have a decent gravity beer 1.060 or higher worth of grains, there is usually enough sugar to give it one more good rinsing.) You don't have to worry about tannin extraction or other fears for extended sparging. I have also just poured cool water over the grains, to rinse them more. One extra gallon is all you are looking to get.
sorry if i'm rambling here, but I just really believe in this, mostly because I'm cheap and it is stupid easy.