My own reading of the subject (grand total of one book) puts the odds against the Cambodians. They simply had neighbors that were too powerful and their own population base does not have the historically ingrained learning to pull any rabbits out of the hat (like new inventions, big exports, or whatever). One may need to wank the Mongols in China and have them decimate Vietnam, Siam, and Thailand for separate reasons and somehow leave Cambodia alone (or, Cambodia being conquered, is turned into some sort of puppet state that is easily manipulable because they are weak). Maybe in this power-vacuum scenario, if enough time goes by with no one overtaking them, they can maybe make Khmer the written language of all south-east asian commerce. Perhaps along the way they strike an alliance with the Spanish who become to weak to subjugate them, but not weak enough in Europe that someone colonizes the Khmer Empire. By 1900, they'd be incredibly backward due to their alliance with the Spanish, but can probably be a second-tier Japan of sorts (or a better version of Thailand OTL in the early 20th century) being that they'd be intact but not all that modernized. Perhaps, due to butterflies, Great Britain sees the Khmer Empire as a buffer between themselves and Japan, and give significant trade concessions and help build them up. Perhaps in this role as a buffer (to ring in ATL Japan, China, whatever), the Khmer essentially will be close to Australians, Koreans, or whomever else is useful to western foreign policy in the east--as a buffer.