It always fascinates me how a dog can hold themselves for so long. Sometimes my dog will wake up, go out and poop and then come bouncing in the house like that's all she had to do. I have to say, "Go wet!" and then she looks at me like, "oh yeah! I knew I forgot something!"
It could actually be psychological, as it sounds like she's gotten mixed signals (through no fault of your own, but she sees 3 ways to pee - on pee pads, anywhere in the house and oh yes, outside.) I think you're doing good by being patient and walking her several times, so she gets the idea that there is only one acceptable place to go.
Anyway, have you taken your little one to the vet to be checked out in regard to this specific issue?
Since she's little, she could be going here and there in the house and maybe you don't see it. If she goes in her kennel, it may be dry the next day. Disgusting thought, but you may want to put your nose at her bed.
Something tells me she's going and you just don't know it. If she's not, you would probably have seen signs of illness in some other way by now.
I wish you luck. I adopted my dog after a year of her living outside and was told I would never housebreak her. That was ridiculous. You have to overcome the previous owner's issues, and again I wish you luck.
Please let us know what comes of this.