wouldn't you only need to deorbit if you "missed"? it's not like we're trying to safely land on the surface of the sun. we don't need it slow down at all, just aim straight for the middle and accelerate in.
As Earth is in orbit around the sun, everything launched from Earth automatically is also in orbit around the sun. To go into the sun, you need to deorbit from that orbit. Earth's orbit around the Sun has an orbital speed of about 30 km/s, so you need to cancel that to drop into the sun.
It doesn't have to get there fast. So what would the effect of putting up solar sails to create some drag to slow down? Wouldn't need heavy fuel to generate delta-v. Would the expense of a system to deploy/unfurl a solar sail cost more than fuel powered thrust?
Aside from the practical concerns with solar sails, there's a fundamental problem with using them to get to the Sun. To lower your orbit, you need to exert thrust in the retrograde direction (directly opposite to the direction you're going). However, that's perpendicular to the direction of the sunlight you receive, so a solar sail cannot generate any thrust in that direction!
once you've negated the delta-v, reel in the sails or detach them or whatever. at that point, Sol's gravity should do the rest. i'm sure some smart people with slide rules could determine when to do it to not be influenced by Venus/Mercury on the way in.
So if you had the sail at a 45 degree angle to the sun, bouncing the photons forward to your orbit, wouldn't that eventually slow you down? Or do you get pushed outward into a slower orbit at the same time and cancel it out?
It's all a moot point anyway since solar sails are so ridiculously impractical that this would basically never work in real life regardless.