As long as her health is good, I would just continue to allow her to nest, but keep an eye on it. This can be obviously physcologically damaging to her if it goes to far. Ideally, she'll stop on her own. For all intense and purposes I would say she's having a "false pregnancy" which usually does not occur to a spayed female; if you haven't already, you may want to call and specifically talk to your vet about this and get their thoughts/opinions. Like I say, I wouldn't stop this behavior or try to innterupt it. Yet.
Other than that, keep doing what you're doing - showering with individual affection/walks, etc.
She may just need time to adjust to the change of your home with Maddox for whatever reason.