21 day trip next May - Circumnavigation

Looking at the Shetlands- in May are you just looking for an external view of Sumburgh Lighthouse, or to also go to the Visitor Centre. If the latter I'm not sure that if you've driven down from Fraserburgh that day, the flight timings are going to allow time to go inside.
Re- the 2 days Lerwick and Bressay, I think I might look at re-organising that a bit. Maybe bright and early on the Ferry to Bressay, drive across island to the Noss ferry landing, then do the ancestral stuff on the way back.
You should be back in time to do some of your Lerwick list.
That way if something happened to the Bressay ferry next day it doesn't impact on the journey to Unst.
Then day 2 do whatever remains of your Lerwick itinerary (the Town Hall, aka City Hall, while worth doing, isn't a long visit. IMHO Shetland Museum is a must, I also hope there is an evening event on at Mareel while you are there). Then early afternoon to Unst.

I would be nervous about driving down from Unst to make a same day flight- just in case of weather or technical issues on the ferries. If you must do it and can stomach the risk then fine. It's a calculated risk. To buy myself more time if it's a night when the overnight ferry to Aberdeen is leaving direct at 1900 (rather than 1730 via Orkney), it might be worth thinking about that. Of course you run the risk of that being weather cancelled but it takes a lot to cancel Northlink. If they cancel then the Island ferries will certainly have been cancelled earlier in the day.

By the way I am sure some/many will counsel against driving on arrival day. If you believe you can manage that, on what is a c45 minute drive, fine.
But maybe think about staying overnight at the airport (at the PI or the Holiday Inn Express) then renting the car early the next day.
Partly depends on your arrival time at ABZ.

Good input. We're only going to Sumburgh Lighthouse to play with the puffins, not go inside the lighthouse. Shetland is the one place we've been on this trip, and we spent a lot of time at the excellent museum last time. The archivist was very helpful with my wife's ancestry.

If we did the Noss walk, it would be instead of the Noss Boat, and we'd visit the home ruin on the same day. Our last trip was fouled up by rain on the planned day for either (and the boat broke down).

I agree about the risk for the last day on Unst, but I haven't figured out a way to eliminate it without adding a day. If we did the same trip, but anti-clockwise, we could get to Shetland earlier in the day and go straight to Unst since we'd be coming from Stonehaven. That raises the issue getting all the things done on the way to Fraserburgh, given our jetlag. Cruden Bay would be easy, but the next day to Nairn would be monumentally long or we'd have to add the dreaded day.

The softest target for recovering a day is to skip the whisky walk and only spend two nights on Islay. Nothing wrong with drinking whisky in the bar (sounds like a good song name).

We've always driven on the arrival day without problems, even if I don't sleep on the flight. As I age, I'm trying to keep the drive shorter, but I did Dublin to Clonakilty a few years ago (and won't do that again!). Just did Dublin to Belfast a few weeks ago, and I was running out of figurative gas by arrival. I'm a little obsessive about not wasting valuable nights.

If you like history and since you will be in the Cruden Bay area, you might want to stop by St. James Church, which is about a mile north of Cruden Bay. It was the site of a battle between the Scots and King Canute's son (and fellow Danes) in 1012. The church came later but inside the church is a font that dates back to that 11th century time. I stopped by the church was closed (I was there fairly early). But I think it would be worth seeing.