Given that a swarm is handled as a single creature, would it be possible for a character with Beast Friendship to take said swarm as a single henchman?
Only if he wears it
https://38.media.tumblr.com/f4b0fedc173b0019cc0edb6706cf002e/tumblr_nphanb2CR71svojj1o4_500.gif
No. It's treated as a single creature for simplicity / mechanics, but it is not actually a single creature.
That said, the legend of the Pied Piper certainly makes this arguable, so if you want to run with it for your campaign, go for it!