If you want to resume a render, you need to save the MXS, because that's where all the geometry and materials are stored. The MXI file is a 2D image, you can't resume rendering just with it. When you render straight out of Maya, it saves a temporary MXS which is deleted after you close Maxwell. If you want to keep the MXS for resuming later, you need to enable "Persistent MXS" in the render options. If the MXS Path field is empty, the plug-in will save the MXS in the same path and with the same name as the output image, but with the MXS extension.
Region rendering now uses the leftRegion, rightRegion, topRegion, bottomRegion and useRenderRegion attributes of the defaultRenderGlobals node, like any other Maya renderer. These attributes are not exposed in the common settings UI for some reason, but we'll add them in the Maxwell render tab or re-add them on the camera, like before, for better control.