First of all let me say Wow!, somebody likes their castles.
The trip is certainly do-able but that is a lot of driving.
When are you planning to make this trip? This year?
Is there any chance of extending your holiday to Scotland to possibly 14 nights?
You have missed out several excellent castles like Glamis Castle, Blair Castle, Scone Palace & Doune to name just a few. These are excellent to visit and everyone totally different to the next.
If you are planning on making this trip in 2019 then you are probably too late to book accommodation on Skye or Glen Coe! For accommodation on Skye it is recommended that you book as early as possible. Many people book, 8 months to a year, in advance.
Make sure that you book your accommodation in advance and DO NOT try to be spontaneous! You may not find any available accommodation.
Hope that helps a little.