Whisk 3 tablespoons oil, vinegar, and mustard together in large bowl. Add celery, parsley, and shallot and toss to combine. Season with salt and pepper to taste; set aside for serving.

Place triple layer of paper towels on large plate and arrange 6 slices bacon in single layer on towels. Top with 3 more paper towels, remaining 6 slices bacon, and second plate. Microwave until bacon is slightly shriveled but still pliable, about 4 minutes.

Toss scallops with remaining 2 tablespoons oil, salt, and pepper. Press 2 scallops together, side to side, and wrap sides of bundle with 1 slice bacon. Thread onto 12-inch metal skewer through bacon. Repeat with remaining scallops and bacon, threading 3 bundles onto each of 4 skewers.

4A. For a charcoal grill Open bottom vent completely. Light large chimney starter filled with charcoal briquettes (6 quarts). When top coals are partially covered with ash, pour two-thirds evenly over half of grill, then pour remaining coals over other half of grill. Set cooking grate in place, cover, and open lid vent completely. Heat grill until hot, about 5 minutes.

4B. For 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.

Clean and oil cooking grate. Place kebabs bacon side down on cooler side of grill. Cook (covered if using gas) until bacon is crisp on first side, about 4 minutes. Flip kebabs onto other bacon side and cook until crisp, about 4 minutes. Flip kebabs so one flat side of scallops is facing down and move to hotter side of grill. Grill until both sides of scallops are firm and centers are opaque, about 4 minutes. Serve with salad.