Janet,
On something as small as Solitary Tree you would not need the bottom water soluble stabilizer in addition to the tear away. Also, as you get more comfortable with thread painting, you will move the hoop in a smoother motion and some of the distortion will disappear. Most first time "thread painters" move the hoop too fast and create long loopy stitches. This results in more stitches in a given space and thus more distortion. The lengthwise grain has the most stability so for example, if you are going to be thread painting a tree trunk, try having the stitches on the trunk follow the grain of the fabric. In thread painting the design (or fabric) will shrink in the direction of the stitches and stretch in the opposite direction. I didn't get into this in the videos because it is more information that most want to know or adsorb at first.
If you happen to have a Bernina that has a hook on the bobbin, thread the hook. This will create more drag on the thread coming out of the bobbin case which is what you want - that way hopefully the bobbin thread won't pop to the top. Assuming that you didn't tighten your bobbin too much, then that should not be a problem. What you might try next time is loosening the top tension (go towards 0 not to 0) and threading the hook on the bobbin case before adjusting the bobbin tension. And yes, I never use anything but a 6" hoop. The larger the surface inside the hoop the more distortion you will have.
I have a new book coming out, Thread Painting with Style, that you might want to take a look at. It should be out in the next 30 days. In the meantime my first book Simple Thread Painting is still out. I hope this helps. Let me know if you have any more questions.
Nancy