Previous installments can be found here. There will be spoilers. “An historian” is a perfectly acceptable Commonwealth convention, haters to the left [side of the road]. Lady Rose, blonde, pink, and charming, is the embodiment of the "English rose" ideal. In Sybil’s absence, she’s also become Downton’s main force of multiculturalism. Last season, she enjoyed a flirtation with Jack Ross, the black American jazz singer. But that relationship was a leap too far, apparently. This…