I separate iPhone screens all day long and yes it is somewhat easy, there’s most likely no way to get a wire in between the glass / LCD. I also fix iPhones all the time and there are a lot of people who think I’m going to do what you said, just take the glass off and put a new glass over it. But nah it’s more simple and efficient to just take the entire assembly off. Especially in the X models with only one to two pieces needed to be transferred over, rather than 10+ in previous models.
Are you transferring over the little plastic pieces that hold the Proximity sensor in place? You did say full assembly so I'm not sure if that also means the little plastic things, but they will keep it in the right spot I believe.
This is unrelated slightly but in any repair on phones with touch id, if you managed to rip or tear the home button. Are you basically left to tell the person who owns the phone, yea your phones fixed but your touch id will no longer work? I always thought you could just swap it with a new button if it breaks in some way. I thought it was only iPhone 7's that was true. Good thing it's not difficult whatsoever to take a home button out
Could be an issue with the battery, Only thing that would make sense. Not sure what you mean by playing up with no photos but I had the issue with the apple logo screen on my iPhone SE. I replaced my battery and it appeared to fix it. I could be wrong though, i just know when you have a phone for a long time the battery tends to die quicker or not charge right.
If your phone was untouched prior to repair, It's very likely they messed up your phone in multiple ways. If you're not aware, The Touch ID on iphone 7's has a connection to the original phone it came from, so if they possibly tore your Home button cable and replaced it with a new home button, it could be why your Touch ID (finger print) no longer works. They also could have messed up your microphone and screen during the repair. I'm not an expert really.. but for sure if your phone had never been opened prior to going to this shop, they definitely are the cause of it. That's an absolute terrible repair shop and I would complain beyond belief until they solved my problem, I really can't imagine how you're feeling now. So sorry Veronica.