Cook 2 pounds of the fresh and/or frozen berries in a large pan over medium-low heat until berries begin to release juice, about 3 minutes. Increase heat to medium-high and cook, stirring frequently, until thick and jam-like, about 25 minutes. You should have about 2 cups of liquid in the end.
Combine lemon juice, water, and gelatin in small bowl. Let stand until gelatin is softened and mixture has thickened, about 5 minutes. Stir gelatin mixture, sugar, and salt into cooked berry mixture and return to simmer, about 2 minutes. Transfer to bowl and cool to room temperature, about 30 minutes.
Fold the fresh berries into filling. Spread evenly in pie shell and refrigerate until set, about 4 hours. (Filled pie can be refrigerated for 24 hours.)
With electric mixer on medium speed, beat cream cheese, sugar, and vanilla until smooth, about 30 seconds. With mixer running, add cream and whip until stiff peaks form, about 2 minutes.
Serve pie with whipped cream topping.
Notes
To make with all fresh berries: Cook 2 pounds of fresh berries (that have been hulled and sliced) with the sugar. Crush the berries lightly with a potato masher to help release the juices before cooking them for the full 25 minutes.
Make the pie crust a bit ahead of time and keep covered in the fridge until you're ready to roll out and put in dish with cooled pie filling.
The pie should be set after about 4 hours in the fridge.