Re: "The detached sail will accelerate but the probe will decelerate"
The trick of using part of the sail to decelerate the remainder was devised by Robert Forward, and detailed in his novel Rocheworld about a crewed voyage to Barnard's Star.
He even found a way to bring the ship back again, still using only the Earth-based lasers.
His scientific paper describing the system seems to be behind a paywall, but here's the abstract