Please do not consider changing the pH artificially as it will be too hard to control fluctuations. The fluctuations will kill your fish much more easily than letting them get used to the high pH on his own. Just be sure to
acclimate him carefully to the water and he will do fine. High pH can cause some problems but once you get fish into a tank and plants sometimes the pH will change some so you need to give it some time. I know my pH came down once I had plants and fish in the tank for a while.
Yes you need water conditioner to remove the harmful
chlorine and
chloramine and heavy metals that are found in tap water. The only time it is not needed is if you fill your tank completely with bottle SPRING WATER. Do not use distilled or drinking water in jugs, only spring water if you choose to use them.
No bettas do not have any more sensitive eyes than other fish and if you have a small cave and some plants and he gets sleepy or tired he can get away from the light if he chooses. No 15 watts is not too bright. I am guessing that you have an incandescent bulb that came with the tank, I would recommend getting a 10 watt mini fluorescent bulb made for aquariums at Walmart to replace it. It will run cooler and save you a lot of temperature problems later. The incandescent bulbs heat up the tank during the day and then the tank temperature drops at night and can make your fish sick.
Depending on the size of the tank it can take longer than this for a tank to cycle. Don't get discouraged, it will take time but be Oh So worth it when it is done. Good luck and congratulations!
Rose