In a 2- to 3-quart pan over high heat, bring broth to a boil. Add shrimp, cover pan, and remove from heat. Let stand until shrimp are barely opaque but still moist-looking in center of thickest part (cut to test), 1 to 2 minutes. With a slotted spoon, transfer shrimp to a board.
Return pan with broth to high heat. When boiling again, add salmon, cover, and remove from heat. Let stand until fish is barely opaque but still moist-looking in center of thickest part (cut to test), 8 to 9 minutes. With slotted spoon, transfer to board. Pour broth into a 1-quart glass measure or a bowl; rinse and dry pan.
Set pan over medium-high heat and add 1/4 cup butter; when melted, add onion and stir often until limp, 10 to 15 minutes. Sprinkle flour over onion and stir for 1 minute. Remove from heat and whisk in reserved broth and the coconut milk, then whisk over medium-high heat until mixture boils and thickens, 8 to 10 minutes. Remove from heat. Add salt and pepper to taste.
Meanwhile, remove any bits of shell from the crab; put crab into a bowl. When shrimp and salmon are cool enough to handle, cut shrimp into 1/2-inch chunks and with a fork, flake salmon; put these into bowl with crab. Mix in 1 1/2 cups coconut sauce.
Lay one crêpe flat in a lightly buttered 9- by 13-inch pan. Spoon about 1/3 cup of the seafood mixture onto half the crêpe, spreading to within 1 inch of edge; fold bare half of crêpe over filling, then fold in half again to form a triangle. Repeat to fill remaining crêpes, slightly overlapping in a single layer in pan. Cover and chill up to 1 day.
Uncover and bake in a 350° regular or convection oven until crêpes are hot in the center, 35 to 40 minutes.
Spoon remaining coconut sauce evenly onto dinner plates. With a wide spatula, transfer crêpes to plates, setting in sauce. Sprinkle cilantro evenly over crêpes. Add more salt and pepper to taste.