I wouldn’t say its very difficult personally, but it can be if you haven’t done it before. Make sure that you are opening it up from the correct side and don’t tear the cable as you open it up. Use plenty of heat and playing cards or picks, and make sure there aren’t any bends or breaks in the casing before laying down the new screen or it won’t fit quite right. Hope that helps!

When removing a label (if it's just a sticker) it's always best to use some sort of heat gun or blow dryer to loosen the adhesive just enough so that you can either peel it off or (this is my preferred method) use a metal pry tool to separate it from the casing.

I found a couple things you could try here https://www.technobezz.com/fix-poor-battery-life-lg-g5/ but it all seems to be pretty basic stuff to help the battery. Another problem could be the charging port, but I would suggest the tips in the link before trying to replace anything else.

Bad LCD. Since the phone is still vibrating and getting calls and texts, then you know the phone is still working under the hood. A new screen assembly should take care of the problem.