You have a lot of good questions... I can give my opinion on some, although haven't restored a canoe myself.
As I understand it, JH Rushton was a wooden boat builder but succumbed to the canvas canoe craze at the encouragement of his son. Most Rushton boats are all-wood. I believe there were some canvas Indians, but all Indian Girls and American Beauties were wood/canvas.
Fiberglass isn't a good idea for a canoe that was built to be canvas-covered. There are several discussions about this... and about removal... and a good place to start is to use the "search" function above and see what comes up. A bad glassing-job equals more smiles for you and your dad. A good glassing-job puts you on the same pedestal as Howard (you'll run into his posts using "search"!).
Use "search" to investigate stripping the interior too... but I believe the consensus is "if you can job it out, do that". Otherwise, think in terms of brain damage versus a great interior (most of us would give up a few brain cells for a great canoe). But if you can do the work outside, you may save your brain.
One way to measure a canoe is to take two sticks or boards, and have someone hold one vertically against the bow-end and someone else hold one vertically against the stern-end (holding as plumb as possible) while you measure between them.
Your next question is one of those "opinion" things... you ask, "For the decks, one is cracked and will need replaced, will losing the original dimenish the value of the canoe or does anyone really care if a deck is original or replaced?"
An antique canoe will have its best "value" (that word is loaded) if all parts are original and there is no damage. So, as a canoe departs from "perfect condition", so does "value" to collectors. However, yours is a family canoe, and as such has value only your family can attach to it.
My personal opinion regarding decks is this: with short decks, I prefer the original one on the canoe even if it has a few problems... but if only half is there-- if damage makes it appear entirely different from the way it's supposed to look-- then a well-executed reproduction fills the bill. One good deck can serve as a template for one needing replacement.
Look at it this way: when you're in a canoe, where do your eyes fall? Much of the time, I find my eyes on the deck of the canoe... well, I paddle up front. I like to think of the others who looked at that same deck. And in the case of your canoe, it would be your relatives who looked at the canoe's deck. So, it's rather cool, knowing your grandfather had his eyes on that same place, half a century ago.
Seat caning: use the traditional 7-step pattern on the seats of an IG... you can find the instructions on line or in several different books. Cane is available on line. I get it from the H.H. Perkins company. Again, using "search" should bring up some inspirational past discussions.
I won't answer the seat-hanging question because Dan or Michael or someone who really knows the answer should handle that one.
Your canoe has open gunwales, I believe, and ours doesn't. I believe the outwale is wider than the inwale and tapers toward the bow and stern, and the outside edge is rounded, but someone who really knows the dimensions, etc., will have to answer this.
Your questions indicate that you are taking care to bring the ol' girl back as well as anyone could... hope some of this helps!
Kathy