Answer to Question 1
As North Atlantic water subducts, it is replaced by warm water from the South Atlantic. If the formation of deep water slows down, so could the transport of warm currents from the south. The lack of incoming warm weather could lead to a readvance of glaciers. It is thought that the Younger Dryas could have been caused by a slowdown of the formation of North Atlantic deep water.
Answer to Question 2
The best orbital configuration for a glaciation would be to have mild summers that do not lead to significant melt-off from the winter snow. This way, the glaciers can continue to accumulate snow and ice, growing annually in size. Northern hemisphere glaciation is favored when eccentricity is large, the precessional state is such that summer occurs at aphelion, and obliquity is at a minimum.
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