Separate the eggs, place the whites in a large bowl and refrigerate. Place the yolks and powdered sugar in a heatproof bowl set over a pan of simmering water and whisk until thick. Set the bowl over a bowl of ice water and whisk occasionally until chilled.
In another bowl, beat the cream until stiff and fold into the cold custard. Whisk the egg whites with the lemon juice until stiff and fold the beaten egg whites into the custard. Drain the peaches and puree. Fold into the custard along with half of the candied violets and half the nonpariels.
Lightly oil a terrine mold and line with plastic wrap leaving an overhang on all sides. Spoon the mixture into the terrine, cover with foil and freeze at least 6 hours.
Ten minutes before serving, remove the mold from the freezer. Dip the bottom of the terrine briefly in hot water, remove the foil and invert the semifreddo onto a platter. Remove the plastic wrap.
Slice the terrine (dip the knife briefly in hot water) and serve with the remaining candied violets and nonpareils.