Don't use the pH stuff.. all you are doing is causing a fluctuation which is deadly for the fish.. your betta will adjust to whatever pH you have normally. Some fish require certain pH, but not bettas.. messing with it can be detrimental to their health.
If your schedule doesn't allow 2x weekly changes, you could get away with 1 100% every 5 days without any issues.. yes, 1 50% and 1 100% per week for a 1g with no filter is ideal.. but it's not set in stone.
Keep in mind a tank 1-4 gallons require the same care - same amount of water changes regardless of filter or not.. you are just giving the fish more room, not making cleaning easier.
70 degrees is too cold for a tropical fish such as a betta. Lower temp water such as that will cause lethargy, slow metabolism, weakened immune system, etc.. ideally 78-82 for these guys.
You can only kill your fish with water conditioner if you put in enough for a 50g tank into a 1g tank.. unless your friend dumped in the whole bottle, it wasn't the conditioner that killed the fish. Conditioner is a must when you use anything but well water and even then it depends on the well.
As for the color change - natural when fish are brought home to change colors.. dulling in color is a sign of trouble though - too cold of water, too much ammonia (due to not using the proper water conditioner), your pH is going wacky because of what you are using is stressing the fish out..
Basically what you need is a heater - for 1-3 gallons a 25 watt is preferred.. and you also need water conditioner - make sure it removes/detoxifies ammonia, nitrates, and if you can, binds metal.
I know it's all been said before, but there are must haves when caring for fish.. even for bettas who have a reputation for being a "puddle" fish that don't need anything. Don't listen to the people at the stores, as they tell you a general care that oftentimes is deadly.. rarely will you find one who knows how to truly keep certain fish. I would listen to some of the people here as some have many years in keeping/breeding these fish and truly know what is needed to properly care for them.
Good luck on everything..