Maybe she's punishing you for making her wear "wee wee pads"?
(Note to self: change career to marketing dog-related products.)
If I remember rightly, you've had this dog for a couple of weeks now? That being the case, again, it's not that unusual, and not always indicative of the dog's long-term behaviour, so the first thing I will say is relax.
I've certainly adopted a dog before that has been this way for a few weeks while she settled in and then she just stopped. And she was the only dog in the house. I wonder if your dog is spending time marking against your other dog's scent? No idea if that is the case, just throwing out thoughts...
I don't advocate the newspaper method, and so, for me, you are pretty much doing all that you can do already.
Make sure that the dog gets frequent opportunities to relieve itself outside.
Watch very very carefully, what your dog does before it toilets. There will be clues, regardless of how small they might be.
If it's possible, (no idea of the weather in your area) leave the door open as much as is possible, so that the dog spends more time outdoors.
I don't find it unusual that she spends upto fifteen minutes to go when you let her out. Your property is still quite new to her, and so there will be lots to sniff and explore. And yes, this can often be a higher priority than going to the toilet, for a young dog.
Sometimes, it can be as simple as timing. After nine months, I pretty much know now when my dog will need to go out: first thing in the morning, mid morning, after we return from our lunchtime walk, around 19:00 and then just before bed. These times work best for when she eats (and therefore drinks the most). If I let her out at other times, I too would be waiting an age.
I'm pretty sure that most of your issues are just "new dog" issues, and that it'll all come good in the end.
At that age, the dog shouldn't really need to be toileting too much. Is she toileting excessively, would you say? if so, has she been checked over by a vet, just to rule out any potential physical issues.