Both SE-editions of Monkey Island are SCUMM-games (note that ScummVM is different from SCUMM, even though ScummVM has a reimplementation of the SCUMM-engine, along with a quite large bunch of other engines).
ScummVM supports the SCUMM-versions up to and including Curse of Monkey Island, whereas the SE-games seem to be using a new variation on the SCUMM-engine, which among other things, adds usage of DirectX file formats, pixel shaders and other modern graphics stuff.
So, no, ScummVM does not support the SCUMM-version used in the SEs, although there has been some work done on support, it is not even sure that it will be feasible to add full support, given how the game relies on i.e. pixel shaders. Since ScummVM is ported to quite a few platforms that does not, and will never support such things, pixel shaders will forever be problematic.
So no, you can't get the source code (Disney owns that).