I have a pump in the tank with a foam prefilter sponge and the output goes under the substrate to a vertical spray bar. With the tank being 8' long and CO2 line inserted in the intake I got small bubbles but did get good enough diffusion of the gas. Currently the gas line is in the sump pump's intake so the CO2 travels 12' to the spray bar and I get just about the same amount of tiny bubbles. You may want to use a needlewheel pump for a mist rather than bubbles since your gas won't be in the plumbing for 8-12'!
My cheap and old pump vibrates a good deal. Perhaps it is unhappy running upside down? Your plan isn't doing that, you are planning to run it on its side. You may want to wrap it with foam to see if that helps the noise.
I love to make spraybars and have used vertical and horizontal top and bottom ones in my tank. Currently I have clear ones that are vertical, see the last couple pages of my journal linked in my signature. I disliked the low horizontal one as plant growth killed the water movement very quickly. You can see the tank so plumbed in my first journal. I ended up using a couple powerheads for water movement which looks worse than a simple spraybar.
Beating Back the Jungle.... was bringing back from the dead
You may prefer a discharge that widens like the ADA lily pipes to reduce flow instead. Perhaps your ferns can be on wood a bit above the spraybar which could shadow it and not block water movement? Just don't plant tall stems in front of a low spraybar!
Look at the ADA video channel on youtube. Plenty of tiny fish in high flow tanks there. Fish like swimming against a current, that is how food arrives. Here is a random one I saved [BGA in hairgrass, gone at the next update] see how the fish are staying at the bottom looking for goodies to arrive in the tiny tank starting at about 2:25?
The fern leaves will cover the sponge just fine. Perhaps you could make your own cover custom carved to remove the straight lines so it blends in the tank better.
Outside the tank might be a bit cleaner with just the heater and pump power cords coming over the edge and you will only need to disguise the CO2 tank but inside the tank it might look cluttered. Prefilter sponges are big, even in my huge tank it looks big. As you are planning to use it mostly to protect the pump's innards possibly you can reduce the size. How are you hiding the heater?
If I planned the same sort of thing I would probably chose to make a vertical spraybar or simple outflow facing forward in one rear corner. At the right height you get the surface ripple and the water bounces off the front of the tank to reduce the flow nicely. The foam prefilter would then be horizontal near the substrate level with the heater just above or below it horizontal as well. Over and in front I could have my tangle of wood covered with ferns and such to disguise it in the shadows. I would be sure to check that the sponge can be removed and reinstalled with the wood in place!
More of my two cents worth. Not a fan of less biological filtration. I ran my sump with just a prefilter sponge and had repeated GW episodes. With huge sponges in the sump I [crossing my fingers and knocking on wood] haven't had that particular plague since. Consider lighting, perhaps my bright light is the issue.