Lilypad,
Hopefully you will come back and be able to take on board some of the great advice given here. There are lots of SMs on MN, with an awful lot of experience between them, of both good and bad relationships. It's good that you are considering problems and how to deal with them before they arise - its much easier to do that rather than be caught on the hop. Many a time I've been a rabbit in the headlights
. My relationship with my DSD (who is now almost 17) is very good, and that is despite her mother only actually talking to me twice in over 10 years!
When my DSD went through her 'not washing stage' we dealt with it quite simply. Not at first - DH became shouty at first, but soon realised it was getting him nowhere.
In the end it was all quite subtle.
Without pushing, we allowed her to wash, or not, but made a point that we expected her to do so. When she chose not to I (and her father) would make a point of 'actions have consequences'. When she tried to cuddle up to me on the sofa I would say "Sorry DSD would you move down, I'm afraid you smell" (she did). When she asked if she could sleep in with us / her dad he would say "only if you are clean". When she wanted her hair done/complained about me hurting her when brushing I'd point out that if her hair was clean it wouldn't hurt. All very simple.
None of it was nasty, none of it was shouty. We were just matter of fact about it. If she said "Mum doesn't make me wash" or similar DH would just say "Her house her rules, my house my rules" - and in actual fact we knew the same battle was being waged at home. There was no critisism of her mother, implied or otherwise (and really, do 7 year olds think like that?)
It took a couple of months, but in those months she wasn't happy with they way things were going. She didn't like not being able to cuddle up to me watching TV. She didn't like not being able to cuddle dad in bed. She didn't like me not doing her hair. She knew that all she had to do was wash - and she worked it out for herself. I think giving her the option was what worked best for us.
You may need to play the long game, and it may not go as you would want next week, but if the option is there, if the bathroom is stocked and the only reason she can give is because she "Doesn't want to" she may surprise you. 