Haha Julia, we may not have control over things that happen to us. But we can control how we react to things. I'm working on this personally as sometimes I feel I don't agree with a lot of issues in my job. But I can choose how I respond to this. If I didn't I'd be struggling a lot in my job with all the things thrown at me, not from my school I should add but the powers that are beyond that.
So, change your mindset to the plucking, instead of looking at it as he's losing his gorgeous feathers, perhaps think that the plucking has brought him some relief and helped him through a difficult moment. Are we putting a human way of thinking on it, and seeing it as self harm? It might not be, just some feathers that are causing him stress whatever reason that may be. Who are we to say it's not right because it doesn't look 'normal'. For Archie, it might be a way of coping with something. I'm not saying I'm on to anything here, just that you can look at the plucking in a different way. I know one thing though, it's not affecting him like it affects you.
apologies here also as I'm aware I've no experience of a plucker and might well feel the same way you do if it happened to me but looking at it from an outsider, he's gorgeous and happy, and you can't ask for much more than that.