This is moisture from the autoclave getting into the backlight. This happens with the 6s onwards because the adhesive apple uses on the backlight isn't strong enough to hold the 3d touch panel. The adhesive is spongy and you can actually wiggle the backlight side to side. To prevent this you can autoclave for less time or put the screen in a food saver bag. The lines will fade over time. A day or 2 they will be gone or hardly visible.
The screen is an exact fit, if there is a dent stopping it from fitting straight then it's not going to fit properly. There's so much you can get away with but if it's bad it won't fit. Like Rany said g-tool is the best bet or use some pliers and protect the housing as much as you can cause it's likely to destroy it. Sometimes I shed parts of the bezel off with a razor blade to make it fit. It doesn't look any different.
This is not a diy repair. Not only will all the equipment cost more then the phone but it's likely you will break the phone in the process. If you can post internationally I would message @jessabethany and see if this is something she would be able to offer you. Not sure if they do international postage but worth a shot as she's your beat option if you have nobody in your country who can carry out the repair
There is no way Apple can tell if you have opened the phone as long as you don't break anything or strip the bottom screws. Apple will only know you've opened it if screws are badly stripped and insulation tape etc have been removed and reused.