The only way that Whatsapp let you back up your chats is using by linking a Google account, but I’m not sure is there’s some workaround like a third parties apps that let you back up your chats.
I know that using a third parties apps to this purpose is unsafe, but I’m just giving an example.
Yes you can! Your offline backup is inside the app data (you need a file manager to access the folder). If you use a Samsung Device, i think you won’t be able to see it unless you plug the phone on your PC.