Your statement about the interference of Square Form practice with transitions in Large Frame Tai Chi that does not rely on internal discipline may certainly be true. One can readily understand where the overlap of square form will naturally interfere with transitions of movements that are largely dependent on the external movements of arms and legs. Large Frame square form however obviously relies on outstretched arms, large swings of the arms at the shoulders and pushing with the legs. Large Frame round form also relies on those same outstretched swinging arms and pushing with the legs with no attendant circulation of internal energy. However...What can be internal in Large Frame anyway? As Wu Chien Chuan said: Every movement in Tai Chi Form has to have two complementary parts of the body, a moving part (Yang) and a stationary part (Yin). When the yin-yang junction is located in the torso of the body, it is an internal move. When it is outside the torso it is external,
Compact square form practice has other roles which do not concern itself with transition . The major reasoning behind Compact square form has to do with teaching the student how to delineate yin and yang in the body. That is to teach the student how to hold one part of the body still while the other part moves. As we have said in this article and others, the exraneous motion of arms and legs is another word for nerve signals along the path of qi. A minor role of Compact Square form in part has to do with movements that call for steadying arms and hands that are so subject to localized nerve impulses.
Indicative of localized nerve impulses is the natural and easier tendency of students to move extraneously from the arms, shoulders and push from the legs. This easier tendency fits nicely within the domain of large frame Tai Chi with its external motions, but it is still not internal. Qi of course traverses the torso as well as the arms and legs. With internal motion where the yin/yang junction is located in the torso, where there is no attendant motion at the joints of the body, the qi will traverse the body unimpeded. With external motion the qi will be impeded at the localized yin yang junctions of the shoulders, elbows, knees, hips, etc.
Practicing transition with transition in mind is a minor task of Compact Frame round form. The major role is to develop internal energy in the practitioner. Only when external movements are truly minimized as in Compact Frame round form can internal movement flourish. Then and only then will such playing of the form will result in seamless transitions. This is because the internal energy is circulating in the body in a continuous manner without any break.
Certainly Tai Chi form movements along with the transitions between them make more sense if they are corroborated with martial art origin of the movements. The student understands why there are such sequence, transition and the meaning of why the body has to be concerned with positioning and impeccable timing. Further study of transition, positioning and timing can then be relegated to push hands and sparring exercises. Once internal energy is attained throughout the body however, there is not the concern with transition that accompanies Large Frame because in Compact Frame the energy is circulating continuously . Where then is any seam for an opponent to attack?