So lessons learned.
Those bolts are designed to break free (iho): I should have been working them back and forth. Proved that to myself when I had the remaining piece of one held by vice grips. Came out easily once I worked it a bit.
Pribaly true for all old bolts: work them back and forth a bit when taking them out.
Also learnt that when using a tap: if it starts to resist, pull it out, redrill, maybe anneal, don't just force it.
I ened up with the one sheild just attached with two bolts, nothing I can really do about it. With luck it won't ring like a bell at certain revvs or road types.

 

And also thank you all for the help/moral support!

 

My rear axle mudplate has one bolt missing also, sheared off too. I suppose when you have the wheel off again the only other option is to drill right through and fit a longish nut and bolt, preferably stainless steel.

 

Would have done that, but the shattered tap in the middle of the hole put the kibosh on that. That's some hard steel!
Thought about another hole somewhere beside it, but didn't seem worth doing.

 

Don't even consider drilling out a broken screw tap, it's rock hard steel like lathe turning bits. Yours, like mine, is destined to rattle if the road's rough and we'll learn to live with it. As a matter of interest, I had the rear wheels off yesterday and it appears there are only three bolts holding the mud plate on, I may be wrong or just my bad eyesight