Answers:

1- Sorry there's no text box in PSE. Maybe if we all lobby Adobe they'll finally put it into the product. LOL So yes, just use returns when you want to go to a new line.
2- Well, there's several things that you can try. You could use the clone tool with a soft round brush to copy portions of the texture onto the seam. It could take some fussig with but I can usually get fairly seemless result. Also when you brought the texture in you could have just resized it to fit your LO. Alot of textures you don't even have to keep their aspect ratio the same when you resize them. But it's really whichever way YOU like.
3- Typically if your photo looks good at 100% in PSE it will fine when printed out. By good I mean... no pixelation or odd artifacts in the image. This is assuming that you're working at a high resolution (300 dpi).
HTH! And holler if you have more questions!