Lightly toast the ancho chiles, either over the burner of a gas stove, or in a large saute pan set over high heat. The skin should just start to blister. Remove from the heat and place in a bowl. Cover with hot water and let soak for 20 minutes. Drain, reserving the soaking water. Remove the stems and seeds and discard.
Cover a baking sheet with aluminum foil. Lightly coat the aluminum foil with vegetable oil. Place the tomatoes, cut-side down, on the foil with the onion chunks and garlic cloves. Roast until tender and the skin on the tomatoes begins to blacken, about 15 to 20 minutes. Remove from the oven and cool slightly. Remove the skin from the tomatoes. Place everything in a blender. Add the soaked chiles and 1/4 cup of the soaking water and process until smooth. Add more of the soaking water, if necessary to create a smooth sauce. Add the lime juice, cumin, oregano, and cilantro and pulse to combine. Taste, and season with salt and pepper.
To a large saute pan over high heat, add enough oil to lightly coat the bottom of the pan. Add the diced onion and saute until just translucent, about 2 minutes. Add the shrimp and scallops and cook until the shrimp just begin to turn pink and curl. Be careful not to overcook the seafood at this point, the mixture will be fried later, so it should be slightly undercooked. Add 1/2 to 3/4 cup of the sauce and cook for 3 minutes, stirring constantly. Remove from the heat and place in a bowl. Add the crab and goat cheese and mix well. Fold in the chayote and cilantro
Place 2 cups of the remaining sauce in a saucepan. Add the cream and bring to a simmer. Reduce slightly while you are preparing the chimichangas. Taste, and reseason, if necessary. Excess sauce can be refrigerated for 1 week of frozen for 2 months.
One at a time, place the tortillas on a work surface. Spoon a line of the seafood mixture down the center. Fold in the top and bottom to form a package, then the sides. Roll to seal. Secure shut with toothpicks
Heat several inches of vegetable oil in a deep pot to 360 degrees F. Using tongs, lower 1 burrito at a time into the hot oil, holding it together with the tongs until it is "sealed." Continue cooking until golden brown on all sides, 3 to 5 minutes, depending upon their size.
Drain on paper towels. Garnish with the creamy sauce guacamole, tomatoes, sour cream, cilantro, and onions, and serve hot with Black Beans.
Guacamole: Combine all the ingredients in a small bowl and mash with the back of a fork to combine. Adjust the seasoning, to taste. Transfer to an airtight container and refrigerate until ready to use.