You can try penetrating oil, but it's unlikely it'll be able to get under
the bolts far enough to help lubricate the threads. It can't hurt,
The bolts are usually pretty tight after the rust sets in. You can try to
remove them with a ratchet, reasonably long extension, and a *6-point* 12mm
socket. If they break loose, they'll probably unscrew easily after that.
If they just plain break, that's a problem. It'll make it pretty near
impossible to install a new heat shield (or keep it from rattling if not
all of them break).