Both are equally impossible.
Doubt it. While not common, there are still quite a few decathletes who've run sub 5, and this is after doing 9 other events. They may not be pure 100 guys, but their training is a lot more sprint-oriented than distance-oriented.
I'll take a wild stab at this and say that the 11 flat sprinter needs half a year of distance training to crack 5. The 30 flat 10K guy will get sub 12.5 after a month of sprint and block training.