to me this seems like total bullying tactics. If you have a joint mortgage, then you are BOTH responsible for 100% of it - it's not your fault she choses not to live there is it? Perhaps you could get lodgers to help you pay?
Anyway, if the mortgage defaults and you get a ccj, so will she, and you will lose any equity (if any) in the property when it is repossesed - perhaps you could point this out to her?Please realise this is not legal advice as i am no law person. sounds like you need to get lawyer pronto.
What it boils down to is this
1) do as she asks
2) get tennants in
3)sell house