Fantastic, has to be done. I would book the ferries in advance simply because it is in my make up to do so, however, noticed people buying on the day for both Lewis and Orkney. On my trip the Minch was vomit inducing, however, I believe there was a crossing on a July afternoon in 1942 that went smoothly so you may be OK. As for Orkney, the heritage sites now belong to the casual day tourist not the ModA and therefore if you want to really enjoy these sites its worth one morning having a 6.00 am visit and getting the place to yourself. As you know there are man other sites on Orkney that very few people go to and can be enjoyed any time of day.
Have a great time and return spiritually and morally cleansed