Heat broth in a large pot. Stir in saffron. Pat fish and shrimp dry with paper towels. Sprinkle with salt and let sit 10 minutes. Use a mortar and pestle to mash parsley, garlic, thyme and 1/8 tsp salt into a paste; stir in paprika. Add water if necessary to form a paste.
Heat 6 tbsp of oil in 15" paella pan over medium high heat and quickly brown the fish 1-2 minutes. Do not fully cook. Remove to warm platter. Add remaining 2 tbsp of oil, onion, scallions and bell pepper to paella pan and cook until the vegetables are slightly softened. Raise heat, add tomato and cook until it becomes sauce-like, 2 to 5 minutes. Pour in the hot broth and bring to a boil. Sprinkle the rice evenly across the pan. Boil for 3 minutes, stirring rice and rotating pan occasionally. Add all reserved fish (but not shrimp). Stir in parsley paste. Taste for salt. Do not stir after this point. Lower the heat, continue to simmer until rice is no longer soupy but enough liquid remains to continue cooking the rice (about 10 min.). Add extra liquid if necessary.
Arrange shrimp, clams and mussels over rice, placing edges of mussel and clam shells so they open facing up. Cook, uncovered, for 15-20 minutes until rice is almost done. Remove pan from the heat and cover with foil. Let sit 10 minutes. Garnish with lemon wedges and serve with fresh alioli.