1. Going to leave this in this forum as it's more of a student question.
2. You aren't asked in residency interviews about your health unless you bring it up in conversation or otherwise draw attention to a potential issue. Like you mention it in a personal statement and open the door to questions about your condition and how it affects your life. Once you match, you will likely have to pass a health screen, and if you cannot pass it, you can lose your spot. There are places that may not do health screens or require a pre-employment physical, but the vast majority of places do for liability reasons.
3. The best person to ask about whether the cardiomyopathy would limit career options is the cardiologist managing the condition. Furthermore, this cardiologist will likely need to provide some documentation that you are physically able to do the job once employee health learns of this condition on a new employee health screen.
4. While residency and attendinghood are two different grinds, both are stressful. Procedural specialties tend to have more "active" call, so you do have to look beyond the training period into long term lifestyle of a given field. Most attendings work for 30+ years or so, especially surgeon types.
5. Exposure to various fields in med school can often allow a student to realize if they are "up to" doing a given field. If they struggle to physically get through the day as a student, it is likely to be even more physically demanding with more hours per week and call as a resident. You have to rotate through the fields of interest and see what seems doable, both physically and mentally.
6. See #3.