1) Hard to say. I have friends from HES who are going to top 10/20 programs, but they were simply brilliant (e.g. Oxford scholars). I, on the other hand, really have no chance at a top 10, not that I cared too much. It's a postbacc that accepts nearly everyone who applies, so you're going to have a broad range of undergrad/professional experiences. That said, if your undergrad background makes you competitive for Harvard/Hopkins, HES will absolutely not hold you back from getting in there. Of those I know who've taken the MCAT after the program, almost all have done extraordinarily well.
2) Most students work. Not necessarily in research (after all, you're doing premed requirements, what kind of science background do you have for science professionally?). Some part time, some full time, although pretty much all my friends were full-time, and included working in a social science department at Harvard, a non-profit, a low-income healthcare clinic, public health research, retail, and foodservice. So it really runs the gamut. It doesn't matter too much if you work in the medical or a scientific field, you just need to make sure you have some sort of clinical work, and since Boston must have the most hospitals per capita in the world, that can be found pretty easily.
