I think that looks just fine! Are you trying to get a more square shape? You could try adding black borders to each side to get the size you wanted. If it's to be used as a throw pillow, I don't see anything wrong with it being an odd size. If you're trying to make it a standard size so you could use a pre-made pillow form then you might have to redo. But it is easy to make your own pillow form. Just use fabric to make it the size you need and stuff that. You should be able to find instructions on here somewhere to make a pillow form.
Do a search for tapestry crochet instructions. If you can find some written by Carol Ventura, or check out her books from the library, you'll be on the right track. Filet crochet also uses charts, but I be your hat is tapestry crochet.
You should be perfectly fine with two threads together. It will just make it thicker and bigger. You might have to play with hook size to get the look you want. Give it a try and it if isn't coming out the way you want after 3-4 rounds, frog it!
What I would do is not finish the body all the way. In other words, don't close it up at the top. Then you can either work a few even rounds for a neck and start increasing for the head. Or make a separate head and sew it to the body at that round. So, in that case, it might be easier to start at the feet and work up. Does that help any?
You could try the interfacing if you're really worried about it. But since it's going into a quilt, once you back it and quilt it, you really should be fine. Many, many baby (and other) quilts have been made this way for ages. The only thing I wouldn't do is use any really long straight stitches in the embroidery parts.