I don't think that AmoryBlaine is being pessimistic or trying to act like a department head, so perhaps you are being a but touchy in jumping all over him. You posted on a message board asking for advice and thoughts and you got what you asked for. Disagreements and challenging viewpoints are part of SDN. If you do decide to get a mentor, be prepared for this, and I would advise not jumping all over them if they offer you what you perceive as discouraging advice to chew on.
As far as the EM fellowship, I have seen that these were mostly done by people who had been trained in IM or FP and found a job in an ER, but as more EM-residency trained grads entered the job market, in order to maintain their working status they needed the formal training. Before I started medical school, I worked in a hospital that decided to open an EM residency, and the non-EM residency trained staff had to either leave or get the appropriate training to get appointed faculty. I doubt many of these people "wanted" to do the additional training, they had to.
In regards to the EM/FM track, the only program currently accredited is at Christiana Health System in Delaware. According to their website, this is the first year they accepted applications, and the first group of residents will start this July. It may be a bit premature to tout the success of this combined training and claim it's popularity. Give it a few years--this may be the next great thing, but right now no one knows.
There was a thread on here not that long ago about someone who wanted to do FM but also figure out a way to get boarded in Cardiology. Many people gave the advice to that poster that I will offer to you: in the next year or so find what you really want to do (EM, IM, FP, Urology, etc.) and do it well. Trying to work 50 hrs a week in an ER and then trying to maintain an IM clinic beyond that (while possible, as there are 168 hours in a week--I'm not going to argue you on that point) will likely burn you out and spread you too thin to do your jobs well. How will you maintain continuity and availability for your IM/clinic patients? EM docs often have to work rotating shifts, so how will you maintain a predictable clinic schedule so your patients can easily schedule with you? What if they have an emergency or need to speak with you? Are you going to walk away from your critically ill ER patients to call back your IM clinic answering service calls? How many hours a week will you be able to give to a clinic? Not many, and this will inevitably mean that your patients are going to have a very long wait to get in to see you. Who is going to maintain your malpractice? Your EM job will not likely cover your IM responsibilities and vice versa. You are looking at having to take out two separate insurance policies $$$$. You mentioned opening a private practice, but stating you obviously would not be able to take call (which is true)...who do you think would take that job? Do you realize how difficult it would be to hire people to work for you who are willing to take on the entire burden of call without any relief from you? You alluded that you know physicians doing this, why don't you spend more time with them and understand the logistics of what they do, and how they feel about it. As an outsider looking in, it is often difficult to fully perceive what goes on "behind the scenes."
I'm 30 and single as well, and I am doing surgery, so I understand your point about "marrying my job," but there comes a point where enough is enough and one needs time to recharge. Plus, in the early years of med school it is easy to get tantalized by different areas of medicine and want to do them all. But at some point you are going to have to pick one residency position and throw yourself into that 110%. If you do decide to do any one of the combined residency programs, more power to you. Just keep and open mind and a realistic eye turned toward your future career, and remember that an overworked over extended physician is not always best for patient care. Whatever you decide, good luck. Hold on to your work ethic as long as you can!