securing output nozzle of cannister filter-necessary?

Did you add the 2 hose cables which form the hose into a 180 degree turn? That alone should keep it in the water, even if it comes loose. There is also a hose clamp version which came out with the latest design. It uses less suction cups, but forms the 180 degree turn with a single piece of plastic. I don't know which is better, just different ways of doing it. If you are afraid of the suction cup releasing the hose, add a nylon ty-rap around it. The cover/canopy should also not have a large opening, so if anything comes loose, it doesn't have anywhere to go far. Once the hoses are all connected, press the suction cups down. The suction cups will hold it in place. Don't use the suction cups to create a bend which the hose resists, as that will put a constant pressure on the cups, causing them to come loose eventually. hth

Like you say, even if the suction cup does break, the hose will pretty much stay in the tank. Your hood will get a tad wet, but that's what it's there for

Test it by popping it off the suction cups and seeing where it will go. Better to be there when it happens, than discovering the effects later ;~)

The key is to not use the suction cups to hold the hose in a position against it's 'will', and only to use them to keep the hose from moving around.

The weakest link on a canister is the hose to adapter connections (ie: at the top of the canister, connections to shut-off valves, and elbows when used to turn into the tank). When these pop, the results are immediate ;~). The weakest is the elbow to a tank as it usually has a lot of weight and some hose vibration. Your Fluval does not use tank elbows, so this is not a concern for you. I've only had one hose pop off its adapter (it wasn't a Fluval), so now I check hose fittings on all my filters once a year.

The next weak point (generically) is the canister gasket. Usually these fail with a steady drip, but older designs could pop the gasket and run a significant trickle. I think this is a non-issue with the designs currently on the market (thankfully!). My best-in-class and worst-in-class awards both go to Fluval. My Fluval 201 (200 series, now long obsoleted by the 300 and 400 series) would consistently pop it's gasket if I messed with shut-off valves too quickly (it didn't like the sudden increase in internal pressure). To be fair, 20 years later, it's still running a tank for me today, but I have a plastic tub underneath ;~). I have eight of the 400 series running (mostly 304s and 404s) and they have been absolutely water-tight. In terms of equipment, today's hobbyists have it a lot easier (and perhaps us old-timers were a lot more devoted and/or crazy, to put up with everything we did ;~).