Okay, there is potential for a big problem if the single drive fails but can anyone quote an example of this actually happening?
It has happened to at least one TB owner over the years. The drive doesn't actually fail (that would require the gears in the accessory drive box to get trashed, and then you can forget the engine anyway); what happens is that the single cam in the middle of the mag fails.
The single shaft dual mag engines are rare anyway.
No vacuum pump likes being turned backwards
None at all? There are, or have been, types which are rated for up to 50hrs in the reverse direction.
I have also built up an electrically powered vac pump rig (for erecting vacuum horizons) and the capacitor-start motor had no trouble running the vac pump in either direction, at 1300rpm. I can't remember which direction I ended up running it in the end but it worked equally. This was the very common 215CC pump. Certainly the myth that the vanes shear off following any reversal (like turning the prop backwards) is a myth - I ran it for hours in either direction.