Definitely replace the battery first. If this does not solve the issue, try a restore to factory. If all else fails, it could be a board-level issue. I have seen this behavior for the Note 4 and 5 as well.
You cant unless you replace the panel which by the looks of the age of the TV would either be impossible to find a part or not worth it. Those lines are on the LCD panel itself. Youre best bet is to buy another TV. Luckily 40 to 43 inch TV’s are very cheap these days.
The glass on many new phones is VERY difficult if not impossible to replace without breaking the display. You would need to invest in expensive equipment such as a glass separating machine, laminating machine, enclave, UV light and various tools and adhesive. In short, you would be saving money by getting it repaired opposed to buying all this equipment.
You’ll have to replace the whole display. Replacing the glass would do nothing for your image. The amoled panel is broken. Also, most repair shops quote for a whole display regardless if the amoled panel is working or not because it is almost impossible to replace the glass on Samsung curved displays.
I dont like using super glue. I know it’s mostly not popular at all to use, but if you get a gorilla glue micro pen or some type of super glue with a controlled flow use just a tiny dot in the corners and one half way between each corner. The iWatch screen is very light, so a little bit will hold it. I’m not sure why, but I have the same issue you are having about every 10th iWatch I fix.