In a medium bowl, combine pork, scallions, garlic, soy sauce, ginger, salt, oil, and white pepper.

Using your fingers, wet edges of a wrapper with a bit of water. Arrange 1½ teaspoons filling in center of wonton. Take one corner and fold over to opposite corner to form a triangle; seal sides. Bring up two corners of long side of triangle to meet at the top, then press together with a bit of water. Transfer wonton to a plate, then cover with plastic wrap. Repeat with remaining wrappers and filling.

Transfer wontons to refrigerator and chill until ready to use.

Remove heads and shells from shrimp; transfer shells to a large Dutch oven or pot. Devein shrimp; set aside.

Add stock, water, chicken, scallions, ginger, garlic, salt, and MSG, if using, to pot. Bring to a boil, then reduce heat to medium-low, cover, and bring to a simmer. Cook until an instant-read thermometer inserted into thickest part of chicken registers 165º, about 20 to 25 minutes.

Transfer chicken to a cutting board and let cool slightly. Drain soup, discarding aromatics, and return to stove. Chop chicken into ½” pieces.

Bring stock to a boil. Add soy sauce and fish sauce, then reduce heat to medium. Add mushrooms, water chestnuts, shrimp, and wontons. Cook, stirring occasionally, until shrimp are fully pink and tender and wontons float to the top, 3 to 4 minutes.

Add chicken and spinach and cook, stirring, until spinach just begins to wilt, about 1 minute more. Season to taste with salt.

Divide soup among bowls and serve.