I've actually consulted the man behind JFA Exhaust regarding the dual exhaust system before previously and he doesn't really recommend it as we might be losing power if we proceed with it. After all, the L15A VTEC isn't exactly powerful in the first place. But he said we can consider using a single exhaust system but extending an extra pipe to the left side from the muffler which will not interfere with the power system. It's not a dummy, but there would be less flow of exhaust to the left side (assuming, driver side as right)
As for the spoiler, original ones direct from Honda comes with a stiffer torsion spring to counter the extra weight by the spoiler. Those that you install outside does not comes with it. Water leaking into the bonnet would be due to the installation process. Usually the part where the bonnet is drilled and screwed to the spoiler is sealed with silicone to prevent water from leaking into the boot. Original spoiler wouldn't have to worry much about this as there's a "water route" for the leak to travel which will not enter the boot. If you are going to custom your spoiler (depending on the design), try to make use of the original location :)
Cheers! :)