Some things to consider:
1) IM -CCM is getting more competitive, but it's nowhere near cards/GI. We had a significantly larger applicant pool this year, depending on your program leadership and their philosophy, EM may be an uphill battle to get in or may not be.
2) There is an antiquated rule that EM applicants need I think 6 months inpatient medicine rotations, and technically consult services do not count (neither do your ICU rotations in SICU/Neuro ICU/Peds ICU/Neonatal ICU). Now most places don't really care about this, or they'll accept consult services, or will be ok if you get close (say 4-5 months). But there are a few sticklers out there who will not consider you unless you have all 6 months. This rule doesn't apply for anesthesia or surgical fellowships.
3) In light of #2, if you're in a 3-year program you will need to tailor all of your electives to critical care or inpatient medicine services (bleh). If you're in a 4-year program then you will be more able to accomodate this requirement.
Personally I really prefer IM-CCM. I'm graduating this year and will be working in SICU/CVICU/MICU so going to a medicine program doesn't impair your ability to work in a SICU if that's what you want to do - I don't know if the reverse is true or not (it may be for all I know).
Yes but it's probably not worth worrying about. There are more important things to consider, such as your LORs. You'll need good letters from intensivists as well as your program director. Much like residency, great step scores will not necessarily get you in, but terrible step scores might keep you out.
This may be controversial, but I'm going to say yes. Fellowships are going to be in academic centers, and academia tends to prefer academia to community. You will also be more likely to be at a program with a CCM fellowship which will help your electives and getting the needed LORs.