Steve --
Thanks for the thoughts. I appreciate you tossing out some ideas for consideration.
First of all, let me try to address some of your questions about the shaft durability itself, and then I'll move on to "counterweight" discussion.
As for the shaft -- I have ZERO concerns about the durability of the shaft, or the cam lock being used. A HUGE portion of the focus of this design has been on the cam lock itself; I feel that one of the main weaknesses of the stock Equinox shaft exists where the shaft pieces connect. In my opinion, the twist-locks, and the spring button/button hole design, is an inferior shaft locking system, ESPECIALLY for "heavy duty" use. Even with "lighter" use, though, I think the "shaft wobble" issues which have been observed with some of the stock Equinox shafts, are reflective of the weaknesses inherent within this type of shaft-locking system.
My focus from the get-go was a shaft design that would employ an extremely secure/strong clamping cam lock, that would not ONLY eliminate any "shaft wobble" potential, but ALSO would eliminate entirely the need for the "spring button and button hole" design. To me, it was not worth proceeding at all, with this shaft design, if I could not employ a locking system that accomplished an extremely strong/secure connection between the lower and upper shafts.
Upon completing the design, my personal testing of the resulting prototype has confirmed that my objective (extremely secure connection) has been fully achieved, through the clamping cam lock I've chosen to employ with my shaft system.
BUT -- in addition to my own testing, I also sent off a prototype shaft for "field testing," to a very well-known detectorist, whose name nearly all would recognize, and who I consider to be the most technically knowledgeable, intelligent detectorist out there (and who is both a land AND water hunter). Testing/evaluation was conducted both in and out of the surf, and among other things, I specifically asked for a focus on, and feedback regarding, the strength of the cam lock connection. After his evaluation, he stated that the cam lock being employed is, to quote him directly, "200% over-engineered" for this application. In other words, the "design intent" has been met -- and THEN some. Hopefully this helps to address any concerns about the cam lock strength...and it SHOULD also address any concerns about the "button holes wearing down over time," since -- unless a customer SPECIFICALLY requests them -- button holes won't even be INCLUDED on the shaft (the cam lock is SO secure, that they are not even needed).
As for your comment about the shaft "snapping like a chopstick," again I have zero concerns. Carbon-fiber shafts have been used for YEARS on a large number of high-quality detectors, as I'm sure you know (Explorers, E-Tracs, CTX 3030s, just to name a few), and I think we have enough history there to confirm that shaft breakage is a very rare occurrence. Carbon fiber is obviously noted for its strength, and as such is used in applications such as military, aerospace, etc. So I'm not sure why you wondered if shaft breakage would be a concern. In my specific case, I have been selling shafts now for a year, and have not had a single report of shaft breakage -- exactly in line with what I would have expected. Can I say that there is no circumstance under which a shaft could ever break? Of course not. But I can say that it would be an EXTREMELY unusual situation, AND -- if it somehow were to happen, I stand behind my products 100%. You would have a new shaft in your hands within a week, should such a breakage ever occur. This, to me, is simply not a concern.
Now, I'd like to move on to your questions with respect to the counterweights. I will be the first to admit that I am not a water hunter; clearly, a counter-balance system is easiest to design for LAND hunters -- and that's been my focus. Even "on land," there are plenty of variables to consider (which coil is being used, how long is your lower rod extended, etc.), but at least the physics can be calculated more simply, and then tested to confirm, out of the water. Once you get into the water, where characteristics (buoyancy) of the coil, water currents, etc., come into play, it's much more difficult to make a balancing system that works for all. What I can say, though, is this is PART of why I chose to include a screw-on cap, on the weights, so that the amount of weight being used can be adjusted, by the user, as conditions warrant (including "in the water"). And also recall, the counter-balance system is OPTIONAL, such that if a user doesn't think it will be useful for their style of hunting, they have the option not to purchase it; the shaft is of course designed to be fully functional WITHOUT the optional balancing system.
Finally, you asked about whether any temperature-induced contractions in the diameter of the carbon-fiber tubes that would theoretically occur, moving from a warm car to freezing water, would "cause any parts to fail." My answer is no, I do not expect so. After much research into epoxies, lots of experimentation, and adjustment over time, the epoxy I have arrived at, which I use now on all rods/shafts, is a VERY strong but flexible (not brittle) epoxy. As such, this epoxy is designed SPECIFICALLY to handle bonding of two dissimilar materials, whose coefficients of thermal expansion differ. So between this trait of the epoxy being used, and the fact that the cam lock itself is adjustable (with respect to the tension of the clamping lever, via an adjustment screw), I am confident that even rapid temperature changes will not be a cause for concern (and again, I stand behind my products 100% should ANY type of failure ever occur, with one of my rods/shafts).
Hopefully, I've addressed most of your questions. If there is anything else you'd like me to clarify, or discuss, please let me know. I appreciate your assistance with thinking through the design.
Thanks!
Steve
...
In fact, the inside diameter of the counter-weight tubes will be nearly PERFECT for accommodating half-dollars! 
And so, since you are the clad-digging MASTER, you could go ahead and fill that tube up with 63 of the Kennedy half dollars that you've dug, and -- based on some calculations I did just for you -- those 63 halves would make a stack about 5 1/4" long, thus fitting quite well inside a 5 1/2" counterweight tube! AND -- along with allowing you $31.50 in emergency beer run money -- those 63 halves would also be exactly the amount of weight required to achieve "complete/perfect counterbalance" of the 11" coil! 
You now have a "get out of Dodge" coin stash to rival that on your wife's Kirby! 
