My first pack I made was pretty useless and has since been cannibalized for other projects. Your pack looks like it will give you some great trips out of it.
1) The trick my girlfriend taught me that works wonders, especially on silnylon but not so much on canvas, is a glue stick. I just apply a decent bit of glue on each piece then stick them together. The glue will keep the pieces from slipping around much, you won't have to put any extra holes in already pretty delicate fabric, and once you are done, the glue just rubs off. For oxford though, pinning works fine. Also, you may want to give yourself a larger seam allowance. What did you use?
2) My first patterned pack was the G4, and for this pack, the pattern has you put an extra piece of canvas across the width of the backpanel, covering the straps. It works pretty well and I have used it on packs other than that one, but I plan to try just having a seam there on my next pack and seeing if that gives it a more clean look.
3) I've used the tops of ziploc bags before for a rolltop closure on a small running pack. On some packs though, I just don't use anything at all, and I think it works fine.