Link to geology of N Pennines.
http://www.northpennines.org.uk/getmedia.cfm?mediaid=868
Last glaciation
http://eprints.whiterose.ac.uk/915/1/clarkcd2.pdf
A, yes the ice would have followed a pre existing valley.
B The highest parts of N Pennines are believed to remained above the ice in last glaciation forcing the ice to flow from Scotland and at times from the Lake District through the Stainmore gap. Local ice is believed to have formed to East of Cross Fell/Upper Tees area and flowed down the Wear and Tees Valleys.
C The change in River wears flow is further down stream.
From natural England website,
Glaciation also altered drainage patterns, by blocking the original route of the river Wear north, causing it to divert eastwards cutting a new channel through the Magnesian Limestone Plateau to flow into the North Sea at Sunderland. The abandoned course of the pre-glacial Wear is now occupied by the diminutive river Team in the Team valley at Gateshead.
Mark
Teesdale,Co Durham