Territorial, anxious dog or both?

Hi there. I live with an old woman who sometimes pockets some money to work as an informal dogsitter of sorts.

So she got a pretty good amount (half a month's minimum wage where I live) to take care of a bitch for a week. She doesn't accept pissfingers, though this is one of the top 2 questionable dogs she ever accepted. An adult female shih tzu with a teeth pointing out (like this picture) which means it probably came from a puppy mill.

This dog took an instant liking to my pensioner, but not to me. Also we're both females of similar stature and the dog's owner is also a female. So it's not man shy.

Whenever I approach this dog, she retreats and hides herself under a table or wardrobe. If I approach her within a range of 3 meters, she gets further away. When my pensioner is home, the dog follows her around like a shadow (expected behavior for a companion dog I think) but also puts herself between me and my pensioner barks at me when I get near. The dog never barks at me when I'm alone with it.

Whenever I open my bedroom's door, she also begins barking. She doesn't stop barking when she sees it is me. The only time she won't do that is when we're both in the kitchen, which is also our dining room, probably because she hopes I'll throw some scraps from my table (which I never did, but my pensioner does even though I told her not to do it).

She also barks back whenever a dog barks in the street. We live on the third floor of an apartment.

The dog is not bite/scratch oriented. The only time I can go near the bitch is if both her and my pensioner are cuddled together in the sofa or bed. She either won't bark or ceases barking when I sit down in the sofa, but still keeps looking at me like I'm a convicted criminal. She allows herself to be pet, but is visibly uncomfortable when I try to do so, even when I don't pass my hand over her head.

Probably the worst thing though is that she has a piss mat. She usually knows how to use the piss mat, but when no one is looking she will pee in front of my bedroom's door (which is always closed). Our floor is carpeted.

Things I have tried so far that didn't work: Talk to the dog in a "toddler" tone, squat to talk to the dog, lay on the floor to talk to the dog, share space sitting next the dog, pet the dog and then give her some kibble (she wouldn't eat from my hand), put the kibble in front of her and retreat my hand (she would burrow it within the blanket instead of eating), live normally not paying attention to the dog whatsoever.

Dog also really dislikes going out for a walk and hides before being collared.

I'm thinking in buying some treats on a pet store and try the kibble strategy again but with something tastier.