You can pin the stay tape in place during a fitting, or you can refer back to the original measurement in the pattern if things have already stretched and you need to pull the fabric back to what it was. The best quality laces are of course the most expensive, but, fortunately, a little bit goes a long way. I was forced to be clever and creative with my use of it, and the result was beautiful, and just as effective, I'm sure, as a much more lavish use of it would have been. Lace has no grain, so layout can be somewhat flexible, although careful motif placement is essential. Taking your pattern, or your muslin, to the fabric store will give you an indication of how the lace will work with your design.
Gathering plays a small but very important role in the success of the gown.
So often a well-made gown is spoiled by lumpy, uneven, unattractive gathers. It's a shame, because it's so easy to make them beautiful. The first job, which is part of your experimentation process, is to find the proper gathering stitch size. It should be big enough to allow you to gather with reasonable ease, but small enough to hold the gathers in place once you've gathered them. Test the stitch size over bulky seam allowances, especially.