If frozen, thaw the shrimp under cool running water, then peel if needed but keep tails on. Pat shrimp dry and place in a bowl with the 1/2 cup seafood marinade; toss to coat and marinate for 15 minutes.
While shrimp marinates, cook the orzo in a large pot of salted boiling water according to package directions until al dente. Drain in a colander, rinse with cool water, and let drain thoroughly; transfer to a large salad bowl to cool.
Preheat a grill or grill pan to high heat for the corn, then reduce to medium-high for the shrimp after charring the corn.
Remove husks and silks from the corn and brush each ear lightly with oil. Grill the corn 10–12 minutes, rotating every few minutes, until cooked through and charred in spots; set aside to cool briefly.
Thread marinated shrimp onto skewers (soak wooden skewers first if using). Grill shrimp over medium-high heat 2–3 minutes per side, until opaque and just cooked through; avoid overcooking. Remove from heat and let cool slightly.
While shrimp and corn cool, chop the spinach and scallions and add them to the bowl with the orzo. Add the lemon zest.
Cut the kernels off the cooled corn cobs and add them to the orzo mixture. Pour in the 1/4 cup French salad dressing and toss everything to combine; add more dressing to taste if desired.
Either remove shrimp from skewers and arrange on top of the salad or place skewers on each plate when serving.