Regarding Journey into Mystery:
I see that its first nine issues were part of the "Fear Itself" crossover and its next three were bannered under "Shattered Heroes." If I were interested in giving the series a try, would I need anything outside of JiM itself in order to understand its issues under those banners?
No, you can read JiM by itself without the main FI event. The story in JiM runs alongside FI and is certainly influenced by its events, but it is its own complete take on the tale.