First make sure you have SR1 patch applied to Ver 12. It gets rid of a lot of bugs that cause the shutdowns. You need to think in terms of closed objects rather than lines. To build a wildlife scene out of clipart you first need clipart where 1. The objects don't overlap, 2. there are closed chains of nodes. If you can't select and fill an object in CorelDraw it's not a closed object. You need to learn to use the Weld and Trim features in Corel so you can take multiple descreet objects and "weld" them together to make one big object. Download some of the sample CDR files off my CandCNCSupport Yahoo Group site in the Files/Artwork folder. I have both combined and separated types of drawings.
DXF exports from Corel segment the curves into hundreds of tiny line segments. That causes the files to be huge.
Try exporting the finished files as HPGL (PLT) exports and bring them into SheetCAM that way. They at least will have more evenly spaced segments.