Who's the winning pitcher?

Pitcher B becomes pitcher of record and gets W. Credit Pitcher C with a save.

If it were 10-0 after 4 and B gave up 8 runs and then C held the 10-8 lead, I think the rule permits the scorekeeper discretion to give the win to C due to B's not having been effective in a short stint.

Actually Pitcher C gets the win ... precisely for the reason you mentioned, Midlo, involving scorekeeper's discretion.

When starter can't get the win because of pitching only 4 innings, and his team has the lead when he exits and retains the lead, the winning pitcher is the reliever who in the scorekeeper's judgement who was most effective.

Here, B pitched 1 shutout inning and C pitched 4 shutout innings ... so pitcher C would be the most effective -- thus the winning pitcher.