On the swords, it's because they're grown immersed, not submerged. I can't remember what the exact difference is, but I know that when they're in the stores, they're immersed. When you get them home and put them in your tank, they become submerged and grow differently. It also depends on the type of sword it is. All swords sold as "amazon swords" at places like Petsmart and Petco aren't actually necessarily true amazon swords (which are HUGE plants in reality), so they grow differently. Google "amazon sword" and you'll see some pretty spectacular plants.
IDK about the Rena, sorry.
I was told that diffusing CO2 through a canister filter is super simple. All you do is still the end of the tubing from your CO2 unit into the intake of your canister filter. It'll then diffuse through your spray bar and viola! Never heard anything about gas being trapped in the canister itself.