The two way thrust bearing was called the translation unit. If it failed, although you could feather the front prop, the rear prop was uncontrolled, and the dynamics of props are such that it would go fully fine. You would get, not a full blown overspeed, but a fast windmill producing a lot of drag.
Had one catch fire and fail on a Mk 3 in '66. We were heavy and could barely maintain height. Started the Vipers, but at that time they were only able to be run at idle or 100% (that was later changed), and they could not be run at 100 for long. Ended up jettisoning fuel with the Vipers running, which had not been cleared. Geoff Grimsdale was the captain.
The oil line to operate the pitch change came through drillings in the sump of the engine. If a piston failed, the con rod was then likely to chop through the sump, so that both props went fully fine and oversped. Don't know about Pelican 16, but this was I believe the cause of the Culloden Moor crash landing (Pop Gladstone) and the Indian Ocean ditching (Hugh Blake)
The straight bars with teeth were called rack bolts. It was possible for one to fail, in which case the front prop and two blades of the rear prop could be feathered, but the one blade would go fully fine. A slow windmill, but lots of vibration.
To get oil from the rotary oil tank to the rack bolts there were spring operated pistons in cylinders attached to the oil tank. Every 5 hours in flight it was necessary to lubricate the T.U.s. This involved increasing the RPM to 2600, then reducing down to the feathering gate, doing one engine at a time. At high revs the centrifugal force pushed oil into cylinders, at low revs the springs pushed the oil out to the rack bolts.
All this from memory. Now, what did I have for breakfast? Damn, I think I forget to have breakfast.
P.S. As regards prop pitch settings, Joe Ashworth's book quotes Front prop fine pitch 23 degrees, feathered 90degrees, Rear prop 24 fine, 91 feathered.