Complex sentence: Yet I would have "Lord Aberdeen" would mean the then Prime Minister (and "Earl Aberdeen" likewise, all Earls are Lords aren't they?) and should thus follow the lines of BDD's reasoning, wherever that leads us.
Split into simpler sentences: Yet I would have thought "Lord Aberdeen" would be for the sometime Prime Minister (and "Earl Aberdeen" likewise, all Earls are Lords aren't they?). I think we should follow the lines of BDD's reasoning, wherever that leads us.