Is it safe to delete ipsw files?
I think that old .ispws will eventually get deleted, depending on the quantity and version.
I don't think there are any commands
Yes, it shouldn't harm your computer or device in any way.
Yes, by using Felix Burns or if you click restore iPhone, let iTunes download the *.ispw, but eject before it can restore (this worked for me once, but I can't be certain, so don't loose all of your data!).