If you're going to use single ball arms and YS connections to the strobes, then you only need a total of four clamps, as you have two connections from your tray handles to arms, then two more between the arms. I'm not sure how good of an idea it is though, as ball mounts give a good deal more freedom in terms of strobe positioning. My old ST-100 strobes had YS mounts, but I just ran them on ball adapters. If you don't mind spending a little bit more, toss the single ball arm and get three double ball arms instead.
Assuming that by 'Inon Optical D Strobe Cable' you're referring to
this, then yes, it will work, but you will also need the Sea & Sea bushing in order to plug it into your housing. For what it's worth, I use
these cables, and they've performed flawlessly where cheaper single-core cables gave me issues.
O-rings get worn out over time and at some point they do need replacement, but it takes years to reach that point - don't worry about it yet.
You may also want to get some floats - an aluminium housing with glass dome and a pair of strobes is going to be heavy in the water. One option is to use Stix foam floats threaded onto your arms; another is to use carbon fiber floats instead. I've been mixing ladder arms and carbon fiber floats - with ST-100 strobes I was using four ladder arms with dome and four 300g float arms (200x60mm) with macro port, but since I switched to Retra Pros, I've been running two float arms and two ladder arms with the dome, and that setup is just about neutral. Keep in mind that I'm using a polycarbonate housing and acrylic dome, while your housing is metal and your dome is glass, so the buoyancy characteristics of your setup are completely different. I've been considering getting four of
these to always have perfectly neutral buoyancy on my camera and stop changing arms between dives, but I'm not quite ready to drop $400 on that yet.
).
