regarding the question about the Caledonian Sleeper -
Until they are nationalized they are operated by Scot Rail who have settled with the Unions. Unfortunately they can be and are affected by Network Rail, the folks who provide the signalling and maintain the tracks and provide the paths to virtually all trains. Network Rail are still participating in the strikes.
You need to follow the Caledonian Sleeper strike page, known as service alterations. https://www.sleeper.scot/service-alterations/
Currently there is the nationwide strike announced for the 1st of April but that won't affect Caledonian Sleeper because they don't operate on Saturdays anyway. There are not yet any other strike days announced for April. They need to give 14 days notice.
If I were you I'd have a read of the page I linked and see how they are planning to respond to the other days of the upcoming sequence of strikes - it will help you understand what might happen if they call a nation wide strike during the days you need.
You'll see that they handle different days differently.
I hope that helps. I realise that certainty is what you want and what is lacking.