For the ones you posted, I would guess that they are primarily decorative, even if they originally had a more functional purpose. I've seen them on Grand Lakers as well, which I assumed were to deflect spray at speeds you'll never attain with a paddle.
There are two recipes from past editions of the Keewaydin cookbook at the bottom of this site:
https://www.ottertooth.com/keewaydin/K_Misc/cookbook.htm
Yes, it's a great route and not very high traffic. It's best done in the spring because the water gets pretty low in the summer. I did it from Fourth Machias Lake to the coast in May of 2014, and have some notes and photos if they would be helpful. I highly recommend the AMC Maine Rivers guide.
The traditional choices are aluminum billy pots (with bails), and pressed-steel cold handle frying pans. They're definitely thinner than the nice stainless cookware I use at home on a stove, but durable, light, and get the job done.
I haven't done South Cedar Creek, but I did once do the mainstem Congaree River from Cayce to Bates Landing. It's ~55 mi between access points so not a day trip, but it was beautiful and peaceful.
The boiler was installed to run the pumps for the lock that used to exist at the dam, to lift the tan-bark steamer between Pocumcus and Sysladobsis. The boat, called the H.L. Drake, collected hemlock bark along the lakes for use in the leather tannery in Grand Lake Stream.
I can't speak to anything more specialized for those two areas in particular, but both the US Geologic Survey and National Resources Canada topo quad maps are free in the Avenza store, so that's what I generally use.
One thing to keep in mind is how the different options affect the total volume of the hull. If you shorten the length by 10% but leave the width and depth the same, you have 90% of the volume of the original hull. If you shrink all three dimensions by 10%, the volume will be 0.9*0.9*0.9, or 73%...
Old Town used hand-caned seats until the late 1930's, then switched to machine-woven (pressed) cane. More info at the link below.
http://forums.wcha.org/index.php?threads/old-town-cane-seats.17104/
I can't speak to the New River, but the French Broad upstream of Asheville is also a good option if you're looking for a tame river in western NC.
https://frenchbroadpaddle.com/
The Machias River in Washington County is free-flowing from headwaters to the ocean (dams removed a few decades ago), but that might be the only other one.
I think the "little over a mile" that Paddlinhal is referring to is if you take out at the beginning of quickwater, and don't line the first part. Probably depends on the water level and personal preference which option is better.
The portage is about a kilometer - you can line up the stream for a while, and then you take out below some ledges and portage the rest of the way to the dam on the road. The one time I've done it I was thirteen years old and rather green to the ways of portaging, so it was a very long kilometer.
The St. Croix River on the border with Canada is dam-controlled and generally has higher water levels in the late summer than others. The whitewater is on the tame side, but if you like gentle rapids it has lots. It does get pretty crowded in July and August.
The Machias is probably not very doable right now - the portion between Third and Second lakes is pretty bony even with good water, and likely virtually impassable now. Come spring, however, it is a lovely trip and I would highly recommend it.