Meme generators are mostly a way to get someone else's watermark onto your picture. Adding the text yourself avoids that.
White text with a heavy dark outline is the convention for a reason. It survives being placed over a bright sky or a dark shadow without becoming invisible, which plain white or plain black text does not.
Size it for a phone. Most people will see your image at a few hundred pixels wide in a feed, so text that looks reasonable on a laptop can be unreadable where it is actually viewed. When in doubt, go larger.
Keep the lines short. Two short lines read instantly, a paragraph does not get read at all.
No. The image you save is the image you made.
The tool ships with a set of clear fonts suited to captions.
No.