Sri Lankan Airlines is set to join OneWorld sometime this year, they are pretty good regionally. They have nonstops into Colombo from NRT, BKK, KUL, SIN. And connecting flights from PEK, PVG, CAN, HKG that route via BKK. They also do some code-sharing with MH via KUL. And of course, MH has just joined OneWorld. So you might find some possibilities there with AA miles.

Weather--temperature is relatively constant. Sri Lanka has a NE and a SW monsoon which affect the different coasts at opposite times of the year, but going between late January up through April should be OK for nearly everywhere. May starts to get iffier as far as rain goes for some of the popular tourist places on the island, and June I'd plan on rain at same.