in this thread: clueless people.
there are websites that discuss everything. The internet lets people pretend to be experts on anything.
The red cross recommends bleach for emergency water treatment. Bleach has a short shelf life, so use something relatively new. IIRC, bleach is one of the few chemical treatments that zaps pretty much everything, although if you're very concerned you will want to double check me on that.
http://www.redcross.org/portal/site/en/menuitem.d229a5f06620c6052b1ecfbf43181aa0/?vgnextoid=b60461150e8ae110VgnVCM10000089f0870aRCR D
any chemical treatment method is going to take longer to be effective at lower temperatures. That's basic chemistry. You're also going to have a much lower risk of there being a contaminant in water at lower temperatures.
Aqua mira does not effectively kill giardia. Read the label. It's for 'enhancing the flavor of drinking [read: already potable] water and killing odor-causing bacteria' or some such nonsense. The stuff isn't even legal to sell in California because it's a misleading product.
I know of no studies of the long term effects of chemically treating your water.
You can treat with iodine and add in tablets that both deactivate the iodine and make it inert (unable to react with anything).
someone posted a giant, thorough military research project on water treatment methods. It covers the effectiveness of most typical water treatment methods against varieties of biological contaminants.
In the end it is personal preference--there's a give and take to each choice, but generally treating water on the AT is a waste of time. It's nice to have something simple like bleach when you're thirsty and crossing a stream that runs through a cow pasture.
I treat my water almost never, and have only gotten ill once in over 10,000 miles of hiking. I'd rather take 3 days of flagyll every few years than drink chemicals 5 times a day every day.