Don't read too much into the graphichs on NEWT. They are for illustration and not precise to scale representation. You have the correct idea that the focuser must be "near" the bottom with the short ep's and further up with the longer ones. Again, the extra in travel is designed to accomodate variations in ep designs, barlow lenses, etc.
The field stop is that circular retaining ring at the very bottom of most lenses in an ep. I have seen some that used the very bottom edge of the eyepiece barrel as the field stop, and some that had none at all, giving an amorphos blurry edge to the field.
You may be correct in the location of the focal plane in the NEWT program but I'm not sure. That would depend too much on focuser design variables that the program may not accomodate.