Just had mine replaced, again with no heads left on a few as they were too rusty, I did cheat & use a yard and they said they had to grind new sides on to the old head in order to get purchase to unscrew using the spanner, which to my mind sounds easier than trying to take a whole head off & hoping the bolt drops through when you pick her up ! I did have the whole keel dropped and rebedded to make the alignment better but I think (unless you really want to drop the keel) the norm is to do them 1 at a time so the keel doesn't move.
Hope this helps
If you can't get a purchase you can always drill and tap a smaller hole into the top of the bolt, make sure you use a reverse thread. That way you can screw another bolt into it, and when it reaches the bottom of the hole you drilled + tapped you should be able to use it to turn the keel bolt. (By reverse i mean anti-clockwise tightens the thread, clockwise unwinds it)
What model and year? In some instances the bolts have a locking nut/nuts in the midst of the keel; they may be covered by putty and then antifouling. Why not adress Benni after sales service?