Visible crew/equipment: During the dinner scene, in some of the shots facing Junior, the reflection of the boom operator (wearing a watch) holding up the boom pole/mic is visible on the glass, toward the top left side of the screen. Additionally, in two shots facing Junior (POV beside Carmela's right shoulder), we can see part of a black reflector screen (collapsible type) between Junior and Carmela.
Continuity mistake: At dinner, when Meadow gets upset, she leaves the table and heads upstairs, so Carmela leaves the table and follows her daughter. Then, when Livia gets insulted, she tells Junior to take her home, and she too leaves the table. The problem occurs when Tony tells Junior, "You mangia, Uncle Jun', she'll come back," and in the next shot facing Junior, the reflection on the door shows Carmela and Livia are still seated at the table.
Answer: That's not Jimmy, just a look-a-like. Jimmy's short and stout, the guy in "Christopher" episode is tall and medium built. The hair is also different.