Review North Highland Itinerary -

This may well be all the time you have available, but beyond Ullapoool it feels quite fast paced.
I fear that you will be clock watching on the journey from Ullapool to Gills Bay for the Pentland Ferries boat. Especially as this is high season when the road will be at its busiest.
That sector would ideally have been divided into two.
If you're on the late boat from Gills Bay it'll be close to midnight arriving into Kirkwall. The earlier boat you would be struggling to get to on time. And two nights only gives one day in Orkney, which is not really optimal, in an ideal world.
What happens if the only Pentland Ferries ship breaks down again, or (very unlikely in August) becomes stormbound?
The answer really is you would have to do Thurso to Inverness in one day, hoping for capacity on either Northlink route, in a worst case diverting to Aberdeen.
It's the lack of flex that concerns me, as much as the tight timing. In some ways the itinerary would flex better in reverse.

I have the same concern.

How about if it...
16 Sleep in Inverness
Drive to the Island of Skye
17, 18, 19, 20 - Sleep in Staffin Bay for four nights and explore the island.
Drive to John O Groats, Ferry to Orkney
Cut the night Ullapool and have one full day of driving to get to Orkney.
21/22/23 - Stay three nights in Kirkwall, Orkney
Drive to Brora
24 - Stay one night in Brora
25 - Stay one night next to the airport in Inverness.
26 - Fly early the next morning.

Skye to Gills Bay in one day- gosh. That's 3 hours Staffin Bay to Ullapool, 1 3/4 hours Ullapool to Durness- so 5 hours so far, then a further 3 hours through to Gills Bay- that is 8 hours drive time without personal needs stops, lunch (at Durness) and sightseeing stops.
No time at all to divert to Lochinver or Kinlochbervie. Do you have time to do Smoo Cave?- questionable.
The temptation is to say, short cut from Ullapool to Tongue via Lairg and Altnaharra. That saves an hour, and is still, arguably, NC500.
But a year ago when I was on the Altnaharra single track road there was a breakdown on the road, and no-one had anywhere to go for over half an hour until eventually the broken down car was manhandled off the highway. Traffic absolutely everywhere in both directions.
It is not impossible to do the drive but it doesn't feel exactly like a vacation.

Should I only go to the Isle of Skye or to Orkney? Not both.
The drive from Staffin to Orkney on Google's fastest route is more than 7 hours. ugh.

Personally I would try to see either Skye or Orkney but not both. Have you already secured accommodation on Skye? If not then that is likely to be the biggest hurdle to overcome in your plan. A lot of rentals are for 7 or 14 nights and likely to be booked up already for August.

The drive from Skye to Orkney is not fast and includes quite a lot of single track road with passing places. In August this road will be extremely busy and google's suggested timing is highly optimistic as many people don't know how to use passing places, causing blockages. Coupled with the general state of the roads and the number of rental cars getting punctures from snagging their tyres on the rough edges of the road/potholes, personally it's not a journey I would even contemplate in August. My blood pressure wouldn't take it! It will take you between 1.5 hours and 2 hours just to drive from Staffin to the Skye Bridge at Kyle of Lochalsh in August.

Best wishes
Jacqui (Skyegirl)

Something has to give in this- Skye or Orkney.
We're not meant to use the RS mantra of "assume you'll be back", but this plan doesn't realistically work, no matter how it is divided up.
You're not giving Orkney the time it needs, or the NC500 the time it needs, either. You have little or no time to cover for when things go wrong.
I totally believe Jacqui saying it could take 2 hours to get to the Skye Bridge. If it does Ullapool in 3 hours is impossible, and you'll need a PNB/coffee and nerve steadying break at Kyleakin (my own choice on the Skye side of the bridge, rather than Kyle of Lochalsh on the mainland side). That would then translate to an essential lunch etc stop at Ullapool and an afternoon tea (etc) stop at Durness then a dash for the finish post at the ferry.
I also agree with Jacqui that I think my blood pressure would be off the scale. There would certainly be a lot of impolite words mouthed at other road users.
As a day transfer Skye to Orkney via the NC500 is a very sub optimal idea- you would want to be using one of two or three possible routes using the A9 instead if it was essential to do it.
See what you can book on either Skye OR Orkney then recalibrate the plan accordingly. If you can get Skye I would favour keeping Wester Ross and the NC500 (but staying on the mainland) with stops at Ullapool, Durness or Kinlochbervie and Thurso or Wick (or something of that ilk).
Likewise if you can get Orkney keep the NC500 broken up into however many 'spare' nights you have.

Thanks for the feedback!
We have revised our visit.
We will visit the wild Isle of Skye for four days. We rented a house in Staffin. I'm working on renting a car.
Then we plan to take the train (all day) from the Isle of Skye to Edinburgh.
We will be able to visit the beautiful Isle of Skye and historic Edinburgh.

Good choice dropping Orkney ,too much chasing the clock involved and especially as the NW Highlands are well lingering in .
The cafe at Staffin stores is very good.

Hi Grace
If you are arriving in Kyle of Lochalsh by train and wanting to rent a car from there, then you might try Morar Motors. They are a small local business with a depot in Kyle. The train journey will take you via Inverness and where you will change trains for the very scenic line across to Kyle. It will take the best part of a day to do that journey.

Jacqui (Skyegirl)

If you are Kyle go have a look in the bus shelter down by the peir.
It's been "vandalized" by a worldly wise old guy who tours the village on his bike offering advice on meaning of life stuff.
I have met both my last trips in Kyle .

If you want just a taste of Orkney (cathedral, Skara Brae, Ring of Brodgar) there is a very long day trip that you can take from Inverness. We did it a few years ago and thoroughly enjoyed the day - the bus ride was scenic and the small van on Orkney was efficient. A few years later we went back to Orkney for two nights.