Have you been referred to a medical specialist as well as a therapist?
As another person who tics, I'd suspect that this irritating one will pass into a new version soon, but that maybe there will be some therapy that could help her to speed up the process. I'd also think that while it sounds like your therapist is doing a great job, it never hurts to get a second opinion from a neurologist especially if her tics are causing physical pain.
Also, you may have done this already, but do you have anyone in the family who has a tic themselves and can talk your dd through it? Often it runs in families to various levels, it does in mine. Or even find some first person stories online that would help her feel less alone? I've had similar motor tics since I was about 5, and part of the distress when I was younger was due to embarrassment about it, and also a sort of shame that I was somehow 'choosing' it, because from the experience of the person ticcing, it does feel like something you are actively doing... But the compulsion is so overwhelming that avoiding it is just not realistic, and surpressing it is extremely stressful - which of course only leads to more!
As I've got older mine have died down a bit, and I've also become a bit better at channeling my tics into less visually noticeable versions, most of the time (the one upside is that even post-baby, my pelvic floor is spectacular 
). But when you're 8 you can feel somewhere between a victim of your tics, and also an idiot who is doing this to herself. It's a horrible feeling, and it can make the whole anxiety/tic thing into a vicious circle.
Also on the anxiety bit - I absolutely understand wanting to follow every path in making sure your little girl gets the help she needs. But given that tics are neurological in origin, I'd be surprised if the stomach knots turn out to be a magic cure. It makes a lot of sense that osteopathy can work for some people as the massage (or the placebo effect of caring treatment) can reduce anxiety and help with tics. For other kids it helps because 25% of children will experience some level of ticcing that will self-resolve within a few months or under a year, so it will seem to the family that the practitioner fixed it. But I also think that it can be an expensive path for a very uncertain outcome, and could lead your dd to really get her hopes up and be very disappointed.
My personal opinion would be that there are some great complementary type treatments that might help her build her own tools to cope with anxiety and with the tics, whether they resolve soon (hopefully), or stick around a whole lot longer. Mindfulness meditation and/or yoga are brilliant ways for her to get in tune with her own body and feel more calm and control - and there are really fun versions for kids! Or a book on massage could be fun for you to try on each other and even for her to use on herself, to calm and give alternative ways of acknowledging her muscles, even the wibbly ones. You may have some much better suggestions from your therapist too!
Really hope she is feeling better soon, my heart goes out to her and to you as a mum who sounds so supportive.