Who/what was the man assisting Stillsen, especially at the end?

Sonny. Stillson's goon who clearly would become his chief of staff or national security advisor or something like that. Played by Geza Kovacks, a Cronenberg semi regular

I think you're looking a little too deep into things thinking they are anything more than bad men. Especially at the end, when the goon basically turns his back on Stillsen for using a baby as a shield by saying he didn't see the photographer. Shows you right there he isn't all bad and certainly could not be the devil.