A) Your description certainly fits a corncrake; you might also have noted the distinctive tawny upper wing coverts. The dangling legs are as certain an identification feature as any.

Despite the apparently feeble flight, corncrakes are long-distance migrants and the birds that breed in northern Europe winter in sub-Saharan Africa, from the Congo south to South Africa. Your bird was on migration and was most likely from the small Scottish breeding population, though it may have come from Scandinavia. Records of corncrakes on migration are few, but it’s not unusual for them to be flushed by dogs.