I don't have the Petersson book, but on the basis of having looked through it in a bookstore it looks to me like a good one - and appropriate to the period of this particular model.
There's nothing wrong whatever with making footropes out of wire. Lots of modelers do it. Wire is a good material for representing lines that have to hang slack.
Different modelers like to do things in different sequences. I think there's a fairly general consensus, though, that a good way to start is in the same order as the real thing was rigged. Start with the standing rigging of the lower masts, then the standing rigging of the topmasts, then the standing rigging of the topgallant masts, then the running rigging of the lower yards, etc. The jibs and staysails present special problems; you may be tempted to leave them till last, but if you do a lot of other rigging will get in the way. But then, lots of rigging is going to get in the way no matter what.
A buntline is a line running from the bottom (the bunt) of a square sail to the yard. It's used to furl the sail. When the sail is set the buntlines are slacked off. When the sail is full of wind, the buntlines lie on the front surface of it.
The question of tautness in rigging lines is a vexing one. The truth of the matter is that when a ship is under way, a lot of the rigging is slack to one degree or another. Even the standing rigging varies in slackness. (In a fairly heavy breeze the shrouds and backstays on the lee side of the ship are slack, whereas those on the weather side are extremely taut.) If you look at a photo of a latter-day sailing ship, or a painting of an earlier one by one of the old master marine painters, you'll see that almost all the lines of the running rigging have some slack in them. (The obvious exceptions are lines like halyards, which have heavy weights suspended from them.) Some lines, in fact, get sort of wrapped around each other as the ship goes through various evolutions. (Each jib, for instance, has two sheets - one on each side. At any given moment when the sail is set, one of those sheets is draped over the stay that's next inboard along the bowsprit.) The problem confronting the modeler is that (as you found out rigging the footropes) gravity doesn't operate to scale. Thread just doesn't droop like miniature rope.
Some outstanding modelers solve the problem by making all their rigging out of wire. (If I had to give the title "world's greatest ship modeler" to anybody, it would be Donald McNarry. His models are rigged with copper wire, in an incredible variety of diameters, each line having just the right amount of slackness. I can stare at a McNarry model for an hour and still not appreciate it completely.) There seems, however, to be a sort of unwritten convention among most modelers that it's ok to set up the rigging with little or no slack in it. Go to a maritime museum, or look through a book of pictures showing models made by experts, and you won't find much slack in the rigging.
There's also a compromise approach - which you're already taking, and which I happen to like. I derive too much pleasure from working with thread to give it up in favor of wire. (I also know I'll never be able to match Mr. McNarry's work.) In cases like footropes and ratlines, where the lines just have to be slack in order to look right, I use wire. Otherwise I use thread, and set it up taut. Here's an example:
http://gallery.drydockmodels.com/hancock
The truth is that most of the lines on that model are too taut to be realistic. But nobody but me has ever commented on that fact - at least within my hearing.
Hope that helps a little. Good luck.