Yes. Ask the TM near the lockers for the Castle Tour queue. You will see most of the main queue, able to take your time and as many pictures as you want. Near boarding, they will show you to the exit.
We first did the castle tour because we arrived when the attraction was very crowded. Then rode the main attraction later in the day. I'll add one tip -
When you get to the normal end of the Castle Tour, you're about 50 feet away from reaching the Sorting Hat. If you want to walk up the aisle to get closer to the hat for photos, just ask, it's no problem. (At that point, you'll be almost to the loading area of the ride itself.) Then you can backtrack and go out the Tour exit.
Just as an aside, I suspect that the Castle Tour line is what was designed as, and will eventually become, the Express Pass line, maybe a couple years from now when the crowds begin to slow down. When that happens, there will likely not be a Castle Tour any more.
Differences between the two lines? The Castle Tour bypasses the greenhouse, which in the regular line is longer than it is interesting. I think you may also pass through a *different* portrait gallery, but see (mostly?) the same portraits. Otherwise, I think you're just paralleling the main line, passing through the same rooms.
And perhaps it goes without saying - no need to stow your bags to take the Tour. Since a big portion of the queue at busy times is for guests just waiting to get to the lockers, make sure you ask a TM if you're at all uncertain so you don't wait in any unnecessary lines.
But I would not skip it because roller coasters make you sick. It is totally different than anything else! 
