I'd be injecting Combivit every day for several days, I think. Or the vet may suggest a different vitamin/mineral supplement as s/he's familiar with the case. I generally give a hefty daily dose for 1-3 days with any very difficult birth, along with the ABs.
I'd be worried about her not getting any fibre into her rumen - no ideas beyond what you've tried, though.
I don't see that yohurt could hurt, thought it's generally used (along with oil, bicarb and ginger) when the rumen's compacted or in cases of bloat.
Fingers crossed for you all...
You may not want to even think about this at the mo, but I'd assume that after all this, you woudn't breed from Penny again. So uness you'd keep her - or find her another home - as a pet / lawnmower, the aim here is to have her rear her lamb and then, once fat, she'd be going off. Of course it's better for the lamb, and for her, if you can bring this about - but if she's miserable, not rallying, it may be better and less work all round to let her go and bottle-rear the lamb. (And yes I know Wabbit wasn't an easy lamb - but they do all come round
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Only you can make that judgement, i just thought it might be helpful for someone to say it out loud. I feel for you, it's a hard place to be in.