Hi particle;
When i prototype I use the cheapest facsimile of leather I can find. Sometimes it is Denim from old jeans, sometimes it is a yard of some gawd awful upholstery vinyl that no one else wants because it's flaming orange with purple flowers or something weird. Sometimes it's vinyl from the back of a broken chair. For your purposes, I would use the vinyl because the fit and feel would be closer to the final product. But keep in mind that fabric or Vinyl will have it's own characteristics and stretch (or not) differently than leather.
As for welted seams that would depend on personal taste. That said, I've never used welting on a leather project. I tend to go for the most simple way of doing things. I am not sure what you mean by "offset" unless you mean seam allowance. If so that would depend on how thick the item is, they type of stitching, whether or not you are sewing with a machine or by hand. But generally a seam allowance is somewhere around 3/8" to 1/2" in garment sewing.. UNLESS you are sewing a french seam.. in that case it's traditionally a 5/8" total seam allowance... sew 1/4" of the allowance wrong sides together then flip it right sides together and sew 3/8" from the seamed edge. This captures the raw edge in between the second seam allowance. Here's a video to explain it a little better.
Now... there is no rule saying it has to be so the finished french seam is on the inside when complete. You can still sew one with the french seam exposed to the right side of the fabric... in fact it may be a great way to finish a piece that is made from 2oz finished upholstery leather. No edge finishing required. Of course with leather you may need to trim away one of the seam allowances before you flip and capture the edges in the second seam allowance to avoid bulk And of course you don't want to IRON the seam flat on leather.. just moisten and rub with a bone. Hmmm... I might have to try that. :D
As for your fitting her iPad... I've not made one. I would have her trace hers on an opened out cereal box and cut it out... then use something common like Quarters stacked so she can give you an idea of thickness of the thing. If she sends this 'pattern" to you, you can compare it to any pattern or template you have.
I agree that you should use Veg Tan and line it with thin veg tan.