Going to SEA for 3 months

I'm planning to fly to Bangkok on July 1 and then I'll travel through SEA for 3 months. My route is going to be this one:

Bangkok -> Siem Reap (Angkor Wat) -> Phnom Penh -> HCMC -> Hoi An -> Ha Long Bay -> Hanoi -> Luang Prabang -> Chiang Mai -> Bangkok -> Koh Tao -> Koh Phangan -> Krabi -> Penang -> Kuala Lumpur -> Singapore

Do you think this route is possible in 3 months? According to the 'Lonely Planet' travel guide it is manageable in less than 8 weeks but I don't think so. At first I will fly to Bangkok for four days with a one way ticket, then I'll fly to Siem Reap. I wanted to book the flight from my home country because I read that you can get in trouble with the airline, if you can't proof onward travel. Then with buses etc. to Vietnam. When I arrive in Hanoi, I'll fly from there to Luang Prabang (Laos) and am going to stay there for a couple of days. Then I'm taking the plane again to Chiang Mai (Thailand). Will I need proof for onward travel again? I read that September and October are the worst months for traveling in Northern Thailand, Laos and Vietnam, so I hope to be in Southern Thailand or Malaysia by then. If it's not manageable in that time I planned to skip Malaysia and take the plane from Krabi to Singapore. Alternative routes are also welcomed. 😊

Thanks!

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