In a small bowl, combine chicken stock or broth with wine and sprinkle gelatin evenly over surface; set aside. Cut tenderloin in half crosswise and season on all sides with 1 ½ teaspoons kosher salt and ½ teaspoon pepper. Place cornstarch in a fine-mesh strainer, and dust cornstarch over tenderloin halves, lightly coating all sides.

In a 10-inch stainless steel skillet, heat 1 ½ tablespoons oil over high until shimmering. Add pork; cook, turning as needed, until well browned on all sides, 6 to 8 minutes total. Transfer to a large plate; set aside.

Reduce heat to low. To now-empty skillet, add shallot, garlic, and remaining 1 ½ teaspoons oil and cook, stirring constantly, until softened and fragrant, about 30 seconds. Add chicken stock–gelatin mixture, orange marmalade, mustard, and remaining ¼ teaspoon salt and pepper. Bring to a vigorous simmer over medium-high, stirring occasionally. Return pork along with any of its accumulated juices to skillet; cook, turning about every 2 minutes and adjusting heat as needed to maintain a vigorous simmer, until an instant-read thermometer inserted into thickest portion of pork registers 120 to 130°F (49 to 54°C) for medium-rare or 130 to 140°F (54 to 60°C) for medium, 6 to 10 minutes. Transfer pork to a cutting board.

Add rhubarb and butter to sauce in skillet and continue to vigorously simmer over medium-low, adjusting heat as needed, until rhubarb is tender, sauce is thickened and glossy and lightly coats the back of a spoon, 8 to 10 minutes. Remove from heat, and stir in herbs. Return pork to skillet, and turn to coat, then return pork to cutting board.

Slice pork thinly crosswise and serve with rhubarb sauce.