So, we did something similar in April 2015 on the spur of the moment. Ditching our previous plans, we left very early (~8-9 am) from Inverness (taking a lesser known route to Kyle of Lochalsh and then over the Skye Bridge) then followed the A87 all the way up to Portree, then taking the A855 to a road which is not showing a name on Google maps, but which goes through the Quiraing to Uig and connected back to the A87, which we followed back down and onwards towards Inverness via A87 to A82. This route was recommended by the tourist office in Portree (they also have free maps, stop there!), and it was perfect. I'd do it again in a heartbeat, though obviously we'd like to go back and spend much more time on Skye.
It was a massively long day and we were blessed with beautiful weather, but we absolutely do not regret it. It was probably the highlight of our trip, but given it was spring, the sunset around 8 pm, so a little different from October. We also hiked up to the Old Man at the Storr (which took awhile and btw you can see from the road if you drive PAST where everyone parks), stopped at Kilt Rock/Mealt Falls (virtually no walk, you will be able to do this) and several other very beautiful sites. The only thing we didn't get to do was walk to Fairy Pools.
I can verify that even if Eilean Donan is not open, you can stop in the parking lot and take pictures, as we did this right as the sun set (with a ton of other tourists). I wouldn't waste time on Dunvegan when there is much more to be seen.
Hope this helps.