Since I am currently rigging my Model Shipways Niagara, I felt compelled to put in my two cents worth. First off, your Niagara is wonderful and I hope that you get back to it. I can tell you that it does not take much in the way of rigging to bring out the magnificent beauty of the ship.
I have followed your build log very closely to guide me through the hull and deck construction. Your work is exquisite.
I have complained about the rigging instructions in my own log for the very reason you identified at the beginning of this post. Sheet 5 and 6 are indeed hard to follow since all of the sails are overlayed onto the rigging plans. But it caused me to do my homework. I had to learn how a ship works, and how and why a ship is rigged the way it is. Most of that was done on this forum. As a model ship builder thinking about doing future projects, it is an exercise worth the effort.
I decided that I had to make a plan. I decided to do ALL of the standing rigging FIRST. That is all of the shrouds and stays. I decided to start with my bowsprit and foremast , working from the bottom up on the foremast, and inside out on the bowsprit. Parcelling out the standing rigging from the integrated plans is a little easier than it first seems. If you start there, take your time, and like me, refer to the wonderfully descriptive build logs on this forum, I believe you will succeed, and the farther you go, the more skill and confidence you will gain. AND more importantly, your ship will begin to really look great.
The running rigging was a bit more complicated. However, start with a plan. I broke the running rigging into two categories. The lines that attached to masts and yards (lifts, halliards, braces, slings) which I decided to do ALL of, and the lines that attach to sails (sheets, buntlines, clews, reefs). The plans are pretty good when it comes to the running rigging that attaches to the masts and yards. It is there, albeit, buried underneath some of the sail diagrams. As far as the rigging that attaches to sails (when you decide to rig without sails), there is nothing, other than a reference in the instruction manual advising us to tie the ends off on the yards somewhere???
I have seen Niagara builds that totally omit the running rigging that attaches to sails. They look great. I decided to do some of this rigging, focusing on the clews and sheets. i just tied them off on the yards. I don't know it if is right, but it looks ok to me. And it adds another layer of running rigging that looks great on the ship.
I am a true journeyman in this hobby, who makes all kinds of mistakes all of the time. However, if you decide to embark on the rigging on your Niagara, and I truly hope you do, I would be more than happy to help you as best I can. I have tried to take good photos of my rigging experience, and label each of my posts to help future Niagara builders to find a particular part or exercise.
And finally, let me say this. Once you tackle the Niagara rigging plans, and get some experience, you will find ALL future plans, for whatever ship you work on next, to be a lot easier to follow.