Pity on that housing. But keep looking - check out this forum's Classifieds if you haven't already.
On your Canon question, a quick look on the B&H website for the G16 housing says it is fibre-optic compatible. So no problem with the Inon strobe.
It also says the housing gives you access to "all controls"
It's easy to get blinded by science with the arms/tray thing. But essentially it is relatively straight-forward. To attach the strobe you need some form of movable arm(s). Most people use two - an 8" plus a 5" to give lots of flexibility and reach with e.g. wide-angle but the ability to tuck-in the strobe for e.g. macro.
The arms are connected to each other and the strobe by "clamps". Chances are you would need 2 clamps: one to connect the two arms, one to connect the final arm to the strobe. And then you have to connect the arms to something that can connect to the Canon housing - the tray. If you go with Nauticam or a similar housing you might not need a tray as the higher end housings have a ball mount that you can attach your arms to using a third clamp.
It might be an idea to try and pick up a copy of Martin Edge's book on UW Photog - you'll find that really helpful. Or, quicker and cheaper - but very helpful, take a look at the Info Sheets that Reef Photo have on their website. It'll help explain things. But Martin Edge's book is a good investment.
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