Thanks for the link, RBF.
The detached cabin interior trim-panel from Door 1R, which is presumably the forward (service) door on the R/H side, and apparently confirmed as from MH370, suggests that the fuselage did not remain intact with all doors closed. Either the fuselage broke up, or the door opened as a random result of the impact of an unpressurised cabin, or a door-assist (opening) system was triggered, or someone opened the door after a relatively successful ditching. The latter seems unlikely.
Regarding fuselage breakup, this would have exposed most or all of the cabin trim panels to the elements. In that event I suggest that the finding of a door trim-panel would be less likely than panels from the rest of the cabin. It seems likely, therefore that Door 1R opened.