You can overdose on water conditioner but you'd have to overdose a lot. Some can even be used up to 5x dose safely, others are less forgiving.
It is very important that you not place your fish around air vents in your house. It will cause the temperature of his bowl to fluctuate wildly and the fluctuations are very hard on your fish. This is not an adequate way to keep him warm, and in worst case scenarios the constant temp fluctuations could kill him. Move him to an inside wall away from windows, doors and air vents. If there is a particular room in your house that is warmer than others that's fine to keep him there. The temp of his bowl will be about 4 degrees above room temp, so for this reason you must use a heater unless your house is kept a constant 80F+. You need an in tank thermometer like this one: http://www.amazon.com/Marina-Floatin...na+thermometer
to see what his temp really is. And you really do need a heater for him.
Unfrotunetly for a tank that size the only ones available are small low wattage preset heaters that are supposed to bring the temp to a setting and then automatically switch off. The problem with them is there are unreliable and tend to fail and/or overheat the bowl. This is why it's really important to use a thermometer with them and watch them carefully. You also need to give it a test run for 24 hours in water that your betta is not in so that you can make sure it's working correctly and will hold a temp and not overheat. Once you find it's working, you can use it with your betta but you must acclimate him very slowly to the new temp by floating him in a cup of his cooler water in the larger bowl with the warm water for about an hour. That way he heats up slowly.
However, 2 gallons is minimum to keep a betta for a long time and even then two water changes a week are needed - one 50% and one 100%. The 100% should include a thorough rinsing of the gravel and all ornaments in the bowl. With something your size, ideally 3 water changes should be used - 50 50 100, and two at 50% and 100% are absolute minimum. Even then your boy will constantly be exposed to low levels of ammonia eating away at his immune system. With this 2 gallons you can use a good adjustable 25w heater. You also need to test this heater first. To acclimate him to this heater you can use your thermometer to see what the temp is currently in the bowl, then set the heater a few degrees below what the thermometer says. Slowly click it up until it just comes on. From there you can click it up 1 degree every hour up to 5 times, then give him a break. The water should not be raised more than 5 degrees a day. After than you can continue to click it up one degree an hour until it finally reaches about 78-79.
Your pellets aren't good for your betta for a variety of reasons. I suggest you get him either Omega One Betta Buffet pellets or New Life Spectrum Betta - Omega One feed 3-4 split up twice a day. NLS feed 5-8 split up into two or even three feedings. No matter what you feed, make sure you give him one fast day a week with no food.
What does his poop look like?
You need to:
* Increase water changes
* Get him away from the furance vents
* Get him a proper heater
* Ideally get him a larger tank and good adjustable heater
* Get him new proper food
If this doesn't help in a few days then he may need meds.
Here is how to do large water changes without shocking him:
To do a water change, use a little cup like a plastic solo cup - this cup must be only for him and have never been used with soap or other chems. Scoop him up in this cup (keep him in the cup about 1/4 full of water - it doesn't need to be much because he won't be in it for long) and leave him in the cup while you change his water. To do the 50% use a turkey baster - dedicated only to him that has never seen soap or chems - and drag it through the gravel and try to suck as much of the poop out as possible, in addition to 50% of the water. Use a thermometer under the running tap to get it to be the same temp as the water that is normally in his tank. When the thermometer says the flowing tap is the right temp, fill back up his tank. At this point, add the conditioner (dose for how much water you change - if you change half the water you add half gallon worth of conditioner, If you do a 100% water change dose for the full gallon change). Float his plastic cup with him in it in the new water. Slowly add a couple tablespoons of the new water into his cup every 10 minutes for at least an hour. Finally, dump him in gently but try to get as little of the old cup water back into the tank as possible. When you do the weekly 100% you will do mostly the same thing except empty his tank fully and rinse everything in it very well under warm water but never use soaps or chemicals. Once it's fully cleaned/rinsed you can refill it and repeat the cup/acclimate phase.