Re: Calling Isaac Newton...
The same means that propulsed the craft toward the star, would similarly now be braking it, as Alpha Centauri would be exerting a far greater effect in opposition to whatever flickers were coming from Sol.
(Whoops! I was beaten to it. Sorry for the repetition)
On the matter of divergence, it wouldn't be too great a task to drop off the occasional lens en route, so the laser beam can be refocused. The lens too, could have its own mini sails that would give it the ability to keep it aligned.