For spiced pears: Combine first 6 ingredients in large saucepan. Stir over medium-high heat until sugar dissolves. Add pears and bring to boil. Reduce heat to medium, cover, and simmer until pears are just tender when pierced with knife, about 35 minutes. Transfer liquid with pears to large bowl and refrigerate until cold, about 3 hours. Using slotted spoon, transfer pears to plate. Boil poaching liquid in heavy large saucepan over medium-high heat until slightly thickened and reduced to generous 1 1/2 cups, about 15 minutes. Strain into 2-cup measuring cup; discard spices in strainer. Cool. Cover and chill pears and pear syrup until cold.
For mousse: Combine white chocolate, pear brandy, and 1/4 cup water in top of double boiler set over simmering water. Stir until smooth (mixture will be very liquidy). Scrape in seeds from vanilla bean; discard bean. Transfer white chocolate mixture to large bowl; gradually add Mascarpone, whisking until mixture is smooth. Cool Mascarpone mixture until barely lukewarm. Using electric mixer, beat 1 cup Brandy Double Cream in bowl until peaks form. Fold whipped Brandy Double Cream into Mascarpone mixture in 4 additions. Cover and chill white chocolate mousse until set, about 3 hours. DO AHEAD: Pears and mousse can be made 1 day ahead. Keep chilled.
For trifle assembly: Cut pears lengthwise in half and remove cores and stems; cut halves lengthwise into 1/4-inch-thick slices.
Arrange biscuits, rounded sides down, in single layer in bottom of 12-cup trifle dish (about 8 inches in diameter and 5 inches deep), covering bottom completely. Drizzle 5 tablespoons pear syrup evenly over biscuits. Using small offset spatula, spread 1/3 of white chocolate mousse over biscuits, making layer slightly thicker around outer edges of dish to allow mousse to be more visible (center of mousse layer will be thin). Starting at outer edges of dish, place pear slices in single layer with curved edges against sides of dish atop mousse, covering completely. Repeat layering of biscuits, syrup, mousse, and pears 2 more times. Cover with fourth layer of biscuits (some biscuits and pear slices may be left over). Drizzle biscuits evenly with 5 tablespoons syrup. DO AHEAD: Can be made 1 day ahead. Cover and refrigerate trifle and remaining pear syrup separately.
Using electric mixer, beat 2 cups Brandy Double Cream in large bowl until soft peaks form. Add 1/4 cup pear syrup and beat until stiff peaks form. Spread generously across top of trifle. Sprinkle with crystallized ginger. Garnish with chocolate curls. DO AHEAD: Can be made 6 hours ahead. Keep refrigerated. Sift powdered sugar over trifle just before serving.