To make the marinade, in a small bowl, add the tequila, water, 3 tablespoons lime juice, garlic and salt. Whisk until well blended. Transfer 1/2 cup marinade to a small saucepan. Pour remaining marinade into 1 gallon zipper-lock bag, add chicken and toss to coast. Seal bag, pressing out any air and refrigerate for 30 minutes to 1 hour, flipping bag occasionally.
Drain oranges in colander set over a large bowl, reserving juice. In another large bowl, whisk 1/4 cup oil, lime juice, honey, cayenne, salt and pepper; set aside for salad.
Brush avocado halves with remaining tablespoon of oil and season with salt and pepper, as desired. Remove chicken from marinade, let excess marinade drip off and transfer to plate.
To cook using a gas grill, turn all burners to high, cover and heat grill until hot, about 15 minutes. Leave primary burner on high and turn other burner(s) to medium. Oil cooking grate and place chicken on hotter side of grill. Cook, turning as needed, until chicken is nicely charred and registers 160 degrees on an instant-read thermometer, 8 to 12 minutes. Meanwhile, place avocados cut side down on cooler side of grill and cook until lightly charred, 3 to 5 minutes. Transfer chicken and avocados to plate and tent with aluminum foil.
Add drained orange juice to reserved marinade in saucepan. Bring to a simmer over medium-high heat and cook until reduced to 1/4 cup, 3 to 5 minutes; set aside.
Whisk dressing to recombine, then add watercress, pepitas, shallots, and drained oranges and toss gently to coat. Arrange salad on a plate. Peel grilled avocado, slice thin and lay on top of salad. Slice chicken on bias into 1/2-inch thick pieces. Layer chicken over the top and drizzle with reduced marinade. Serve immediately.