Clean mushrooms. Place mushrooms in a small bowl and mix with 1/4 cup marinade. Marinate at least 1 hour (and up to 4) in a sealable bag.
If desired, shell shrimp. In a large heavy-duty sealable plastic bag combine shrimp with 3/4 cup marinade and seal bag, pressing out excess air. Marinate shrimp, chilled, for at least 30 minutes and up to 1 hour. Soak skewers in warm water 30 minutes. Prepare grill.
Drain shrimp, discarding marinade. In a sieve set over a bowl drain mushrooms and reserved mushroom marinade for basting. Thread 3 shrimp and 2 mushrooms, alternating shrimp with mushrooms, and keeping the mushrooms flat onto skewers. Kebabs may be assembled 1 hour ahead and chilled, covered.
Grill kebabs on a lightly oiled rack set 5 to 6 inches over glowing coals, basting with reserved marinade and turning them occasionally, until just cooked through, about 3 minutes on each side. (Alternatively, broil kebabs under a preheated broiler 2 to 3 inches from heat about 2 1/2 minutes on each side.) Drizzle kebabs with remaining 1/4 cup marinade.
Mince together garlic, thyme sprigs, and rosemary with salt and mash to a coarse paste.
In a bowl whisk together garlic paste and remaining ingredients until emulsified. Marinade may be made 1 day ahead and chilled, covered.
Yield: makes about 2 1/2 cups