Do ahead: Toast the pine nuts. Heat a dry frying pan over medium-high heat. Add the whole nuts & toast, stirring often. When the nuts are fragrant with a light brown color, remove them from the heat & the pan. This should take only a couple of minutes. Let the pine nuts cool on a plate or paper towels before using.
Make the lemon-butter sauce in a medium saucepan. Melt 2 tablespoons of the butter and saute the shallot and garlic until they are translucent and soft ... *do not let the vegetables or the butter brown. Add the lemon juice & white wine & reduce by half, then add the heavy cream & reduce again by half. Season with salt & pepper, then add the remaining 14 tablespoons (1 3/4 sticks) butter, whisking it in, piece by piece, over very low heat.
To cook the shrimp, melt the 2 tablespoons butter in a large skillet & saute the garlic until soft. Add the mushrooms, saute for a minute or two, then add the shrimp; cook until they are pink.
Add the spinach by gently folding it in (it just needs to be wilted, not cooked all the way through). Add the toasted pine nuts.
To serve, place the pasta on warmed plates with the shrimp mixture on the side or tossed into the pasta. Pour the lemon butter sauce over the pasta, & drizzle a little over the shrimp - if tossing with the pasta add lemon butter sauce to pasta then toss the shrimp mixture into pasta.