You are correct Pranayam is not "universally safe" for all. It should be perfmred under the guidance of a "realised" teacher and not under the guidance of someone who has not yet mastered it themselves. Sadly many of the teachers and schools fall into the latter.
Originally, students were assessed over a period of 12 years before teachers would initiate them with advanced breathing and posture practice. These were based upon insight of the individual's makeup and dosha balance. The 12 year period, helped each individual work out karmic inbalances in energy (excessive solar) etc before they started amplifying their energy. This is also why many fall by the wayside, as they cannot control their urges, and by using postures and breathing techniques to amplify their already imbalanced energy it leads to issues.
Now in your case, it seems you have a weak constitution due to illness. A few things to note:
1) Pranayam relies on the storehouse of prana around the naval to be "full". Therefore sexual retention is a must.
2) Weakness in the heart centre and strain shows there is a lack of vitallity to support his exercise I would advise you stick to other methods of meditation rather than breath retentive forms of pranayama. Other methods that can help are mantra chanting - start out aloud, the vibration of the mantra will help replenish the upper chakras over time. Breath mantra is also strenuous for many, and it leavs the upper chakras "dry" becausr the flow of energy in their system is more downward oriented than upward. The karma needs to be worked out and burnt without strain or delving and reaching for higher levels before an adequate base has been set.
Hope this helps.
