1. Black background. It'll help the black silicone seams disappear. See any of my DBP tank journals for details about how that looks/helps. Not using a black background on DBP tanks is beyond distracting.
2. Yes, rocks. Especially if you can find large, black lava rock or something similar
3. If you can find the perfect piece(s) that utilizes the height of the tank
4. It's small enough that you can use high-end substrate like Aquasoil with little expense. Could even get a 9 liter bag of Amazonia for $35 shipped right now.
5. If you go canister, consider an Eheim 2213. Gives you a bit of flexibility when it comes to flow. Will be perfect with lily pipes.
6. If you don't go with glass pipes, it should be cheap and easy to create something that looks good. You could even spray paint Eheim pipes black to help them blend in with the tank.
P.S. If you want to keep things extra-simple, you could create a 'carpet' with, say, 20+ pots of Crypt parva. That would allow you to ditch CO2 and go with lower, cheaper lighting.