The same image exists on the E-series boxes... It's confusing to say the least...
I would be 100% confident its fully submersible, where did you read that it wasn't?
Contrary to the markings on the heater itself, the heater is in fact fully submersible. Sorry for the confusion Jason, if you still have the heater, you should be able to safely used it completely submerged in your aquarium.
As far as using it horizontally is concerned, that's fine. As long as it's got enough room beneath it for water to flow all around it it'll be absolutely fine. Just don't go half burying it in the substrate.
Over speccing your heater is commonplace for aquarists... If nothing else it allows temperature changes to be achieved quicker. Efficiency is such a woolly term nowerdays, it needn't really be applied here... The M50 will be as good at turning electrical current into heat as a smaller heater, it'll just do it quicker... Once the tank is up to temp the heater periodically turns itself off and on to maintain temperature so as long as it's powerful enough to reach the desired temperature it won't overheat the water just because it's larger.
Hope this helps..