Orkney Question

Have you considered flying? Loganair flies between Inverness and Kirkwall. I've flown that route and others with Loganair. It's a 45 minute flight.

My suggestion is take the fight up to Kirkwall, spend the night, and return the next day.

If you are going to drive up you are not really going to do justice to the sites in the time available between the morning and afternoon ferries on the same day.
It is a long way (and not to be under estimated) drive to Thurso for either Northlink or Pentland ferries to Orkney. Pentland will take you to St Margarets Hope in the south isles, it is still a good drive from there over the Churchill Barriers and past the Italian Chapel to Kirkwall then out the other side of Kirkwall to the main prehistoric sites.
The better boat, if that is the purpose of your visit, is the Northlink route from Scrabster [Thurso] to Stromness.
I would suggest taking your time on Day 1 to drive up to Thurso- there is a lot to see on the way, and you might as well make a virtue out of the distance.
On Day 2 take the first Northlink ship. Spend the day on Orkney, then book 'Bed and Breakfast' on the ship while she is berthed at Stromness overnight then you are on board when she sails at 0630, and you have all day to drive back south.
Or, if it's a day of the week when Northlink are sailing from Kirkwall at almost midnight to Aberdeen, book on that (with an overnight sleeping cabin) you could do that, arrive in Aberdeen on Day 3 at 0700, it is a much shorter drive back to Inverness from Aberdeen,
The other option on Day 1 is to take the evening ferry to Orkney, and overnight in Stromness (my favourite) or Kirkwall for an hour or two more on Day 2.
If you fly to Kirkwall it is one of those really nice small town airports, and a very nice flight in, as you are flying low at a few thousand feet up- not 30,000. It is the kind of flight, and Loganair are the sort of airline, which brings pleasure back to flying. Don't miss your Tunnock's biscuit on your in flight catering. It's a well known Scottish snack, and is kind of iconic of Loganair. On a good day, like many domestic Scottish air routes, it is a travel experience in it's own right- not just a means of transport.

It sounds like you're renting a car. If so and your rental is allowed on the ferry, taking it across and having it on Orkney could be a very good thing. When I was there, some people rented cars in Kirkwall to drive to the things they wanted to see. I had to do a tour and use the local buses to get around.

I think the NorthLink Ferries route from Scrabster to Stromness is the easiest way to get to Orkney, whether you walk on or drive on. There are sailings daily and each takes about 1 1/2 hours. From Stromness it's a short drive to Kirkwall. If you walk on like I did, a local bus shows up shortly after the ferry docks in Stromess and goes directly to the bus station in the center of Kirkwall.

Here's a link to the Northlink Ferry options: https://www.directferries.co.uk/scrabster_stromness_ferry.htm

At the Tourist Information next to the bus station I arranged a tour for the following day. It included the major sites and it did take all day. We did most of the items on the list linked below that required driving to them. The in-town things I did on my own. This somewhat dated link also has other information you might find useful: https://www.zigzagonearth.com/things-to-do-in-orkney-scotland/

The Direct Ferries link above will give you all the options for the dates you want to go. I noticed that there are daily sailings in July from Scrabster to Stromness at about 13:00, and daily sailings from Stromness to Scrabster at about 11:00. If it was me, and I could take my rental car on the ferry, I'd drive up to Thurso and take the early afternoon sailing to Stromness. I'd drive to Kirkwall and spend 2 nights in Kirkwall so as to have an entire day on Orkney. Then drive back to Stromness to take the late morning ferry to Scrabster.

The Pentland Ferries options look more complicated and I've read that their route to St. Margaret's Hope is often rougher than the NorthLink Ferries one. But you might find their schedules more suitable to your needs. Either is a significant distance from Inverness.

When I went, instead of taking a ferry of any kind back to the mainland, I flew from Kirkwall to Aberdeen. It was a tiny plane and it was very windy. We sort of landed sideways. 🥴

This is the actual Northlink ferries timetable page. Book direct with Northlink.
Departures are from Scrabster (Thurso) at 0845, 1315 and 1900, and from Stromness at 0630, 1100 and 1645-
https://www.northlinkferries.co.uk/booking-info/timetables/#STSC
This is the Bed and Breakfast option I mentioned on board ship-
https://www.northlinkferries.co.uk/booking-info/offers/hamnavoe-bed-and-breakfast/
This is the Pentland ferries website.
https://pentlandferries.co.uk/timetable-2/
They sail daily from Gillls Bay (near John O'Groats) at 0830, 1230, 1630 and 2130,
and from St Margaret's Hope (Orkney) at 0640, 1030, 1430 and 1930
(at least in high summer, reduced service at other times of the year).

The 2130 would get you into Kirkwall after the drive on Orkney at close to midnight, but the hotels are used to dealing with late arrivals off the Aberdeen and Shetland ferries, so as long as they know then it is not a problem.

What I actually came back to say is that, if you decide to come back through Aberdeen, then you can even make a virtue out of a journey.
You can either come back to Inverness via Braemar and Ballater, or take the scenic drive around the north east coast of Scotland, as well as taking the direct and fast route back on the A96.

We flew to/from Kirkwall from Edinburgh last June, and rented a car. We had to bus into Kirkwall due to lack of cabs and arriving after car counter closed, and pick up the car in town the next morning. Three nights was just enough time to see the top items on our list. We had enough time for a Rousay hike, which took most of a day, so two nights might be enough if you hustled and skipped Rousay.

I wrestled with the Pentland ferry approach for many iterations, and every itinerary was dependent on a long drive going smoothly before ferry departure, and the unplanned herds of coo or sheep were too risky.

We flew from Edinburgh instead of the cheaper and faster flight from Inverness because we didn't want two rental cars at once, so when we got to EDI (from Stirling), we went straight to Orkney, then were car-less for Edinburgh afterwards.

Thank all of you for the swift and knowledgeable replies! Just what I needed -- thanks so much!

Please keep an eye on this forum regarding the status of Pentland ferries. Their only vessel is currently out of service following an incident, with relief cover or a date of return to service to be announced