Are there yoga classes in your area? Increased flexibility, and tolerance of holding what at first felt like awkward poses, are among the benefits of yoga exercises. And a lot of yoga postures have potential for bondage, sex and torture; if you learn them, you should demonstrate them to your Mistress and see if they inspire her.

Yes, it's normal to have circulation trouble after holding any pose for a long time, the human body isn't designed to stay rigid. (I was a life drawing model once, and it was a hazard of that job.) You may be able to experiment with slight differences in position to make it easier; but any kind of exercise should also improve your circulation, which will help.