The stolen Strad resurfaced in 2015, several years after the death of the apparent thief, a violinist with a checkered career, when his ex-wife turned it over to the F.B.I. The bureau returned it to Mr. Totenberg’s three daughters, Amy, Jill and Nina Totenberg, who decided to have it restored and sold — but who wanted to make sure it wound up with a musician, not locked away in a collection.