Most species of termites have soldiers.  This is a special caste within the colony that takes on certain roles. Soldiers tend to have large heads.  It is very common for their mandibles to be greatly enlarged.

No.  In general termites prefer timber that is a bit soft, decayed or weathered but they will eat many types of hardwood (wood from trees with flowers not cones).  There are also many types of tree that produce timber which is generally termite resistant and quite a few of these trees are hardwoods.

A few times each year the sky seems to fill with fluttering termites, but not all of the termites get to fly. The colony lets a few grow wings and strike out into the world. A rare time when termites act as individuals. These fliers (called alates) are just for colony reproduction. They seek to find a mate and start a new nest of their own. Just like most of we do.