I think you have tried all reasonable approaches, Tony. I can't see what more you can do. I guess refusing to wash or take a bath can be one way of exerting some kind of control. Seems a bit weird but there is usually a reason behind most human behaviours, unless we are very unwell. A psychologist would explore what she gets out of this and would then ask you to reflect upon what other ways might she gain the same benefit apart from not washing ? Usually for someone to change their behaviour there has to be something in it for them - if only just wanting to please others or 'fit in'.
Does this behaviour not have any negatives as far as she is concerned, eg not being allowed to be involved in certain activities which she would otherwise enjoy ?
Yes, at the end of the day you may have to accept things. We can rarely change others unless they want to change, I guess. ( People like to nag me sometimes but I wont share the details here
Best wishes and don't give up !