something just doesnt sound right.
The last thing I learned in plumbing class was water seeks its own level and the class.
Now in my area of the country the toilet tank sits higher then the bathtub waste.
So in another words If I flushed the toilet and there was a stack/main stoppage water and waste would take the lower path in my case the tub.
I think you need to call a local plumber in your area where it would appear water flows up hill and they know more about this stuff then I do sorry.. Good luck