Diaphragm issues

It is important to not mix medical treatments used within Tong Ren and energetic imbalances worked on within qigong practice. If you can breath well, your diaphragm probably has no problem functioning correctly. Because the energy tends to collect too high in the abdomen/chest area has little or nothing to do with the proper functioning of your bodies nervous system and control of the diaphragm and more to do with the relative level of circulation in the conception vessel meridian and your ability to descend energy in your body.

Personally I wouldn't worry too much about your diaphragm or anything else. The microcosmic orbit will open over time - it does for everyone in it's own time. The main thing is not to force the issue. The more you focus on the energy there, the more you add there. Just do the qigong in a relaxed way, let what happens, happen, do a proper closing and go for a short brisk walk after practice to regulate your energy and smooth your breathing.

If you are going to do any kind of additional visualizations they should most likely be focused on the dantien or even the soles of your feet, not on the area of trouble. This will, over time, remind the body to descend the energy instead of trying to force it through a particular area by collecting more there.

From a tong ren perspective you should be focusing on the points that are valid for your chronic illness and not perceived energetic imbalances from a specific qi gong routine. As your health improves, your energy circulation will improve as well (and vice versa of course).