The whole "nepotism" thing is being HIGHLY oversold here. None of the people I know in derm have relatives in the field. I'm not that sure where you got that from, keeping in mind that much/most of what you hear as a premed is incorrect.
Connections/networking are important, as they are for all the smaller fields. This is how people with weaker apps get matched; they have amazing letters from people big in the field. That's how an individual from my med school matched derm with a low-230s several years ago.
I am in a smaller field (not derm), and I will say this: about half my residency interviewers mentioned to me they personally knew at least one of my letter-writers. That's how you use networking. It's not by being someone's niece. Residency isn't like med school, you aren't getting in based on blood relations or recommendations alone.
To actually answer your questions:
Doesn't matter until you take Step 1, but my recommendation to every pre-clinical student is to keep an open mind and shadow in your free time. People are drawn to the "dollar signs" of derm and plastics, for example, without actually experiencing them until third year. Wanna know what will really kill your perfect ortho application? Realizing you hate the OR. Etc.
Your backup will probably be internal medicine or pathology. You likely aren't going to be able to pull something else off. This planning is complicated and is best left for M4.