5.0 is not so far out of their range that they won't adapt to it. :) If it isn't too far out of the ideal, it's better to allow them to adapt to the new pH rather than fiddle with it chemically, which can cause fluctuations.
You don't want to mess with it.. any chemical you add in there to raise it will work for all of an hour and then it will start lowering once more, which causes fluctuations and that is what will harm your fish and could even kill if it's changes drastically.
Yes, but they aren't controllable. For instance, you can add Texas Holey Rock, but that will raise it quickly and dangerously to a pH of around 8. Honestly, your betta will be just fine with a pH of 5. :)