1. Yes - my first project was a simple rectangular APEX quilt with a triangular footbox made by sewing the two halves of the bottom of the quilt together. I thought it was really, really easy. The WPB edging will make it a little more complicated, though you should still be able to handle it - however, I don't think it's really necessary considering you are intending to use synthetic insulation, unless you are using it for particularly wet conditions.
2. It depends on how much down you would use. The 133g/m^2 APEX should keep you warm to 35 degrees; I would imagine you would need 12-15 oz of 800 FP down for a similarly rated down quilt, depending on the size of the quilt. There's a discussion here - http://thru-hiker.com/projects/down_quilt.php - about how to calculate how much down you would need. You won't win with warmth/weight ratio with APEX - however, it will be much easier to deal with for your first project, cheaper (initially - not in the long run, down lasts longer than synthetic; but initial costs should be your primary consideration if this is not the last quilt you intend to sew), and of course has the benefit of retaining more warmth when wet.
3. Jamie Shortt has some instructions on making a down summer quilt here - http://thru-hiker.com/projects/down_quilt.php - and claims to have done "much experimentation" and has decided his dimensions are the minimal width that is comfortable for an average size man. He is your height and 160 lbs, for reference. I am also your height and a meager 135 lbs and have made some quilts shorter and not as wide as his, and I find them not quite big enough. I think his size would do you good. His quilt is also the same style (velcro foot box) as yours. I think the Nunatak Arc Alpinist comprimises a bit on size for weight, based on reading their description a while ago.
4. No, you do not need downproof fabric. The stuff's magical - you don't even need to sew stitches through the middle, edge stabilization is all you need.
5. Don't worry about it if you're using a bivy or a tent. For a big tarp with no bivy, you probably want windproof/DWR ripstop for the top layer. You can get 2nds 1.1 oz DWR ripstop for really cheap ($4-5/yd, half as much as the popular - and lighter - thru-hiker.com stuff) on diygearsupply.com
6. See 4, nope! Sewn through on the perimeter is just fine (which is why this is ideal for a first project!)
7. My hiking buddy has an M90/Silk 2.5 oz/yd^2 APEX quilt which has such a slit (it almost drapes like a cape, unlike the Jacks R Better wearable quilts. I think it has a big larger dimensions to enhance its use as a wearable insulation layer). It looks pretty awesome. I'm sewing a 20 degree down quilt soon which also utilize the slit; I hope to be able to use it as my primary worn insulation, inspired by Glen Van Peski's commentary on using your sleeping bag/quilt as your main insulation layer, instead of bulking up on warm clothing and carrying a summerweight quilt (more efficient use of warmth - jackets, etc are complicated and require more weight for their warmth). Yeah, velcro closure.