The difference between the two coregistration is that the first one (coregistration) is based on a double cross correlation of images (coarse then finer). It has the advantages of needing no external input.
The DEM-Assisted Coregistration is based on orbit files and DEM. I suggest you to read the following article which clearly explain the method : impact of dem-assisted coregistration on high-resolution sar interferometry
http://ieeexplore.ieee.org/document/5613931/
The idea is to find an analytical expression of the pixel’s shift between the images caused by the topography. It thus requires an external DEM.
For you second question I don’t know. The first method gives you “poor” results. However, if the orbits are not precise enough, the second one cannot help you much.