Hello again David,
I was laid low yesterday recovering from a brief attack of food poisoning so I had plenty of time to contemplate your situation.
As Phil has said, you now have little to lose.
Michael G got there there ahead of me by suggesting filling the underspace with resin mix; the granite version would add very useful weight and damping as if the space was occupied by cast iron.
I would recommend first fitting the front guide block as well as you can, leaving the adhesive to harden fully and drill through into the headstock to increase the depth of the tapping, finishing by opening out the existing tapped hole to a little more than clearance size to prevent any further disturbance of the piece in later bolting down.
A strongback across the underside of the bed at the far end, into which you can fit the two headstock bolts will hold that end down but again I would recommend drilling out the threaded holes in the broken sections of the bed so that they become precision clamping spacers instead and prevent bolting strain on the headstock at that point.
I am so sorry you have had this truly unpleasant experience; was there no sign at all of headstock movement or vibration when the lathe was run up? They could of course have fiddled that with slack belts to hide it.
Best of luck Brian.