Given the conversion to Islam occurred in the 1200-1600s, a time when Islam was resurgent in the Mid East and placing a bloc of Islamic states between Europe and Asia, I think that is the time to start. If the Crusader States and Byzantium were more successful in the 1100s then perhaps the Islamic states would be focused on saving their own skins and have less resources to engage South East Asia with, leaving more of a blank slate for the Western Europeans to play with in the 1500s.
But the ultimate case was the strength of the Islamic states in India and elsewhere, where the crusader states couldn't possibly interfere. That probably means an Islam-screw in India to keep strong Hinduism/Buddhism/native religion (all three were basically fused).