A 5 hour party is a long party, so the guests will eat about a meal's worth of food-

Peel and eat shrimp- (? lbs)
cocktail sauce- how much?
The most hard to manage item will be the shrimp. People are greedy about them- some will take a plateful. I would put them with the chicken, so people will take some of that. too. Also, given the difficulty of the shells, unless this is a sit down party where people can free both hands, I suggest you get peeled, tail-on shrimp. Allow 1/4 cup of sauce and at least 3-4 ounces ready to eat shrimp per person, if you premix the shrimp and sauce together, people eat less; serve in a bowl place inside another bowl of ice, and put out only 1/3 the first hour, divide the rest into 3 and put out each part the next hour. Frozen mussels are much less expensive and could be used for part of the seafood choices.

Bacon wrapped scallops- (? lbs, how much bacon do I get?)
Scallops vary a lot in size! 2 ounces can be 1 scallop or 4! It takes about a half slice of bacon per scallop, you should allow 3 large or 4 medium scallops per person, get thi-sliced bacon, and calculate it after you see the size of your scallops.

sausage stuffed mushrooms (how many mushrooms per person, how much sausage do I get?)
You want two to three mushrooms per person, depends on size, and 2 ounces raw chicken sausage for the stuffing.

chicken satay (how many lbs do I get?)
Allow 4 ounces raw per person
Peanut dipping sauce- (how much?)
Allow 1/4 cup per person

I suggest you add one major vegetarian option, I would do hummus or baba ganouj with pita chips and dipping veggies

I would also add some fruit, at least grapes, 3 ounces per person and cheese (some could be logs, balls, or spreads), 2 ounces per person.

We will have about 3 to 4 other prepackaged items
Allow at least 2 bites per person of each one.