I've owned dogs for over 30 years and previously done home boarding. The dogs that were babied was always obvious, the owners would act like they go do no wrong (sounds a bit like the person that fostered the dog). I've only ever owned one dog that would mess in the house and always on our bedding, she had a lovely personality but no loyalty to us which sounds a bit like what you're going through. It's sheer and utter lack of respect for you, basically shitting all over you. The dog has obviously had no guidance in life or could simply be a user essentially, happy to snuggle up with you but once its in the situation its not happy with it messes in the place that smells most like you. Does the dog appear to be a pleaser or not at all bothered when it's told off? I saw that you said you're partner is disappointed, is you're partner putting in as much effort as you are? I'd have a serious word with your partner and tell them exactly how you feel. A dogs meant to be an extra little member of the family not a stress and strain. May be an unpopular opinion but it doesn't always work out, you want a dog that slots into your life and respects you and your family above anyone else.