No it doesn't, fletching gives +1 range as well as +1 attack, while careening gives +1 armor to ALL ships, not just the fireships or demoships, so it wouldn't help if the galley player also researched careeningMove Careening to feudal, and make it a bit cheaper. There. This cancels out the effect of Fletching which fixes the issue entirely.
Regarding your idea to give fishing ships additional armor, I think that would help, but I still don't think giving demo's extra radius would be good, because it would be too strong against land units