I'm no behaviourist, these are only my ideas drawn from personal experience.....
The neck is a really sensitive / vulnerable area for dogs and so pulling him down by the collar is being (in Pepper's eyes) fairly confrontational. One of my rescue dogs is especially reactive around the neck area and I can guarantee that if I tried to get him out of the boot of a car or off a chair by his collar I would get bitten.
I think you're right in saying that at almost a year old, Pepper has well and truly hit the teenage tantrum phase! Him growling is letting you know that he's not happy about something, if you ignore it and force him to do that thing against his will, what you teach is that its not worth growling, so he is then likely to up his game and move on to mouthing / snapping / and possibly ultimately biting.
That said, you obviously need Pepper to get out of the car boot, get off the chair or whatever when you ask him to. I would, as the other posters have said, try to turn the situation around so that it becomes the best thing ever for Pepper to do what you want him to do.
It may help to have a house lead on him as well so that if you need to, you can lead him off the chair etc without having to physically get close to him. If you do that, I would always reward him doing the behaviour (getting off the chair) even though you're the one that made it happen. That way you're reinforcing that getting off the chair when asked to is a good thing.
Hope this helps