1

1. You'll need a return ticket to obtain a tourist visa.
2. No extensions. You'll have to leave the country for a new visa application
3. Not really, except for mountain regions above 2000m (and air conditioned buses)
4. November

2

In general, weather in South East Asia including Myanmar in September is not cold, so you do not need warm clothes at that time. As for visa, you should check carefully to avoid overstay.

3

1. You shouldn't need a return air ticket to get the Myanmar Visa...(depending on where you apply for the Visa...here in OZ you dont)...and to answer your Q....yes, you can buy an Airasia ticket from within Myanmar using the internet (I usually do this if I am unsure just what date I will be departing the country). (some times the internet there can be very slow, sometimes reasonably fast...if you have trouble try again at a different time or try another internet place).
Risk you take with using c/card on the internet....as with just about anywhere.
2. no Visa extension...but it is possible to overstay and you pay extra $$$ on departure (do a Search for that topic on here for more info...pretty sure Monty did that on his last trip..??))
3. I doubt it. December-Feb are the colder months...in September I doubt you will need warm clothing even in the higher altitudes.
4. November thru to April is the 'high' season.

4

When I went in 2010, with a one way ticket, I bought my outbound ticket 2 days before I left. Wasn't a problem getting in or out. Looks like high season prices are already in effect! Permanenty! Didn't overstay, but lots of people have said it's 3 dollars a day, and not a problem. No personal experience with that.