1

Yep, definitely worthwhile, go for it. Yes it will downpour for an hour or three, maybe on some days it may rain all day. Yes is will be hot and humid, So wear sandles, shorts and a poncho and off you go. There will be less tourists, plenty of accomodation, transport won't be booked out. Get out of Yangon and head North, as there will be less rain. Some things may not be operating, I think like the Balloons at Bagan, and some boats, but it is still an amazing country whatever month.
Re flight. Your cheapest flight option is to Bangkok, then to Yangon or Mandalay. Check out Travel.com or AirAsia to get to Bangkok. If you are concerned about booking online, then printout the cheapest deal and take it to Flight Centre to match it. Then I think AirAsia is cheapest into Myanmar, the further you book ahead, the cheaper with AirAsia. Allow an hour or so to get to AirAsia's other airport in Bangkok.

2

as above...get the cheapest flight to either KL or Bangkok, then Airasia from there to Myanmar (KL to Yangon, and Bangkok to Yangon or Mandalay)...I usually go Air Malaysia form OZ to KL/Bangkok....last time Singapore Airline was the cheapest, from Bris to Singapore...then Airasia-KL-Yangon. Or if you dont mind the less leg room try Airasia from OZ to KL...

3

Bagan is pretty dry area if you want to avoid the rain, Mandalay and Inle are not that wet either, I would avoid Sittwe/Mrauk U at that time and not stay in Yangon long, also the south is wetter. Flights look at to KL and then Air Asia to Yangon or fly to Bangkok and then take Air Asia to Yangon or Mandalay.