If the 2nd curtain problems persist (dark strip), any ideas on other likely culprits ? Old strobes (4 years), worn out fiber optic cables, batteries, other ???
I don't think that old strobes or fiber cables can cause such effect of dark strip. This is exactly looks like wrong synchronization (2-nd curtain is lighted).
Anyway, better solve it step by step. The initial step, - right settings in camera menu (Flash sync speed), then everything will be more clear or totally solved. Easy to check it on land. You have to set 1/200 or 1/250 flash sync speed (without "FP") in camera menu.
Dark strip on images is shutter 2-nd curtain. It became visible in shutter window at the moment of flash, because of inconsistent flash sync speed setting in your camera menu.
Nikon D7100 has mechanical shutter, which makes fully open window at max fast speed 1/250. If you set faster speed (1/320 or faster, or FP speeds), then camera mechanism does not make fully open window at all, but just an open strip which moves along the window (2-nd curtain goes after 1-st curtain faster), the more fast speeds cause more narrow open strip.
Regarding "FP" flash sync speeds in camera menu *
Nikon FP is fast shutter speed shooting method, when camera initiates Nikon land strobe to burst 40khz multi-pulses.
But underwater strobe cannot burst 40 khz FP light, underwater strobe burns only continuously in a single pulse. That is why underwater strobe must burn only at the moment when camera shutter window is fully open, to light the full shot. Such shutter speed, when window is fully open, was named as "X-sync speed" in photography technics.
The maximum x-sync speed which you can set in D7100 menu is 1/250: Bracketing/Flash--->Flash sync speed--->1/250. But, without "FP" !! Only this case underwater strobe will light the full shot.
By the way, If you would use any old DSLR camera, like Nikon D50 for example, (it has electronic-mechanical shutter with x-sync speed up to 1/500), then underwater strobe can work even at 1/500. I tested my strobes with Nikon D50 and #11031 UWTechnics TTL-converter, - all strobes worked perfect in TTL at 1/500 s shutter speed.
Regarding Sea&Sea YS-D1**
The 1/200 restriction in User's Manual, - it is a mistake. This strobe works with all available x-sync speeds (without FP of course), the same as other uw strobes. Believe me, YS-D1 works at 1/250 (without FP), i have just tested it again now. You can set 1/250 flash sync speed in D7100 menu and use YS-D1.
I can only assume that Sea&Sea put 1/200 restriction into User's Manual to avoid any consultations about flash sync speeds, because 1/200 works as x-sync speed for all modern cameras.
.... I did a comparison bench test, using both the Technics board and the onboard strobe taking rapid secession shots. I did not get the same results you describe above, the three shots I took with the onboard flash TTL all seemed to be exposed exactly the same. ...
Do you mean pop-up flash on camera body? On air? There is a difference with underwater usage. On air the short enough pre-flash on Canon TTL system works normally. But water absorption required from developer better to encrease pre-flash to get stable results for Canon TTL metering system, - it plays the main role.
Of course, not all manufacturers pay such attention on TTL accuracy underwater for Canon, and you can find any Canon TTL circuit on the market which has short pre-flash, and hope you can use it for serial shooting with Z240 (3 shots per second), although TTL exposure will be different at each next shot in series.
Mike, i understand that you want to get normal TTL exposure at fast serial shooting camera mode ("Continuos shooting mode"), maybe 3fps - 7fps or faster. You don't like 1-2 seconds waiting for Z240 recharging for getting accurate TTL. i can understand it.
Did you try such fast serial shooting mode with Z240 TTL in the past? Because, using Z240 strobe in TTL mode, Independently of anything, every next shot in fast series has significantly less lighting than previous shot, - Z240 does not have enough time for recharging and it causes different pre-flash (and different main flash) at every next shot. The idea of TTL metering does not work normally for serial shooting with small uw strobes.
People usually shoot in "single shot" camera mode with Z240 TTL to get accurate TTL exposure. This is independently of camera brand and TTL-Converter brand.
But Ok, you can find any TTL-converter which has very short pre-flash (for Nikon, Canon, or other), and get 2-3 shots per second using Z240 this way in series, but pay attention that you cannot get normal TTL exposure in series (there will be very different flash exposure for all 3 shots).