After spending some time, and copious quantities of pink plumbing tape screwing all the fittings together, fitting them to the kegs and getting everything setup, I've come to some realizations that would have made the build-process easier had I known them in advance.
Tapered fittings (such as American NPT) work best for pipe-to-pipe type connections, such as attaching valves to fittings or camlocks. The tapered thread, combined with some thread-tape, locks the fitting in place and prevents leaks without additional seals or gaskets.
However, tapered NPT fittings are less useful when making bulkhead-type connections, such as connecting fittings through the keg walls. Due to the tapered thread, the nut or fitting must tighten exactly as it compresses the gasket or washer against the wall of the vessel However, especially when thread-tape is applied (which is essential to ensure there are no leaks along the thread) the fitting can easily lock-tight too quickly and not seal to the side of the vessel or not tighten quickly enough, potentially allowing for leaks along the not-sealed-thread.
On the other hand parallel threaded fittings (such as British Standard Pipe Parallel thread (BSPP)) work much better as bulk-head connections. Thread-tape prevents any leaks along the thread as the connection tightens securely against the side of the vessel.
With this in mind - and knowing that at 1/2 inch size both BSP and NTP threads mate without problem - bulkhead type connections through the wall of a vessel would be made best with an parallel BSP thread, while fitting-to-fitting connections like the ball valves, are best using tapered NTP thread.
Since the Chinese Ebay supplied lock-nuts have a groove cut in them (which most local fitting suppliers and LHBS usually charge considerably more for), the easiest and cheapest method for sealing through-keg bulkhead seals was simply to use the nut and a silicon o-ring (as per the picture of the thermowell above). However as the fitting is tightened the o-rings can deform or break easily making them less than useful, and essentially useless in any situation that does not have a channeled-nut for them to sit inside. As a result many of the fittings required both a silicon washer and stainless shim-washer so that the fitting would hold tight against the wall of the keg.
While it might reduce the life of the seal, and potentially be the source of contamination (but since the seals are silicon this should not be a problem), most bulkhead fittings were sealed on the inside. This was to stop the water/wort from getting into the threaded connection and between the fitting and the keg-walls, which could happen more easily if the seals were located on the outside of the keg.
With all the fittings attached it was nice to see that the sight-glass worked as expected. The silicon is transparent enough to easily see the water-line, however over time, I expect it will go cloudy so the little floaty-ball should be more useful.