1.) you should put that into plugin. BTW. Using existing 3rd party newsletter plugins is IMHO much better approach, unless you are offering something specific with your own implementation
2.) Read this to learn difference between .PO and .POT
https://www.icanlocalize.com/site/tutorials/how-to-translate-with-gettext-po-and-pot-files/
You can just rename .PO to .POT
3.) Hard to tell from this screenshot, please share screenshot with full lines. But it’s as @Zorbix said