Firstly, I recommend asking this question over at craftster.org, as it's about construction. There are lots more crafters over there, and they're always happy to help.
Secondly, I recently made a challah cloth as a gift that I think was about 12 by 12, and it was patchwork (see it here: http://www.craftster.org/forum/index.php?topic=300793.msg3433349#msg3433349
I used quilters cotton for the top, and backed it with linen - partly because I had some lying around, partly because it was nice and thick so it absorbed the bumps created by the patchwork top. Also, I like my challah cloths to have a bit of grip, I don't like the satin ones, they fall off and get under my wheelchair wheels. So I would say don't bother with interfacing (it might make it too stiff), but choose a more substantial backing than satin. Go with the cotton. Also, press your patchwork very carefully - it might sound obvious but some people don't seem to do it, and it looks very sloppy when they haven't.
I actually embroidered onto a separate piece of cloth that I appliqued onto the patchwork afterwards - partly because I had the embroidered motifs I did while I was in Israel, and wanted to use them, partly because trying to put an embroidery hoop on patchwork would probably distort the fabric. I reckon that if you want to embroider straight on to the patchwork fabric, you should probably do the embroidery before sewing it squares together, but you might want to ask more experienced people over at Craftster. If your embroidered designs run over squares, you'd probably have to do it after sewing the squares together. If you use lightweight cotton and PRESS PRESS PRESS I think you'd be ok sewing through the seam allowance if you are careful and take it slow, and don't stretch the cloth too tight to distort it at the seams.
Hope this helps a little. But do check out Craftster, they know much more than I do