For a 31-week preemie with no complications and normal development as of 2-years-old, how likely are future problems due to prematurity?

Very unlikely. A 31-weeker who had no complications and has been normal to age two years, can be looked upon as a normal person just like the full term kids. We are all human, so there is no perfect child. Any future problems are likely due to being a human, and not due to being a 31-weeker.