I haven't read through what others posters have said but I'm sure you've heard from many that slapping in the second bridling situation wa uncalled for. I even think slapping because the horse reached out was somewhat uncalled for, if this girl gives the horse treats by hand of course he's going to expect her to give him a treat. I only treat after my horse has done something, sadly many others at my barn give him treats when he's standing outside doing nothing so now he expects it from some of them.
In the past my horse was nippy with hands, I asked that no one gave him anything by hand, only in his bucket, it soon solved his nipping. Now if he reaches towards me for treats I hold my hands Palm up towards him to show him there's no treats, he soon gets the message and leaves me alone. He was Incredibly head shy when I got him because his previous owner used to hit him around the head to correct him. It made him a nightmare to work with because even after I'd had him a while if I moved to quickly near his face he'd react with fear, he fractured my jaw once while I was reaching up to put his halter on and he panicked thinking I was going to hit him. Even after this I don't hit him in the face, 10 years later and he can still get funny about people, moving to fast near his head.
I would learn to bridle the horse myself, If you're nervous about getting it wrong have someone take a quick look after the bridle is in to make sure it's right. You have to learn some time, why not now? Also I'd ask the BO to tell the staff/ teens to stop giving your horse treats by hand and to put them in his bucket if they really want to give him stuff, he's your horse, what you say goes.