I'm glad you've got him now
I hope he behaves himself in his bin cage. A gerbil with bed-head makes perfect sense to me, chances are he'll tidy himself up sooner or later.
I'm glad you're a bit more relaxed about his care now.
For what its worth when your friends gerbil partially ate his brother it might be that his brother died naturally and then was eaten. I know it doesn't make it any less icky but as prey animals rodents do sometimes try and clean up after a deceased cagemate and that might be what happened in that case. That said, introductions are never guaranteed to be straightforward so I get why you're reluctant to consider one and I'd never suggest someone start keeping gerbils by getting a lone gerbil and introducing him to another. I wouldn't go by your gut instinct when you've only just met the gerbil though, when we first got Mystery there were signs he'd been fighting in his previous home (where we were told his cagemate had died) yet he's been much happier since his introduction to Fixit. I hope your gerbil is happy as a single gerbil, but it might be best to see how he goes for a few weeks before making a final decision about if you consider an introduction in the future or not.