The only CalMac ship still running to have been built with passenger cabins is LORD OF THE ISLES. I'm not sure when they were taken out of service, but I believe their location doesn't meet modern safety requirements.
As far as I know, the cabins could still be used, but there's really no use for them on the CalMac network. None of the routes really justify cabins, and CalMac isn't really set up to accommodate people staying overnight before an early morning ferry.
particularly the two Aberdeen to Shetland vessels, which are now simply too small in the main tourist season.
In fairness, the Northlink passenger ferries are in really good condition. They've been looked after properly, and were designed for the North run from the very beginning. For that reason, they're not really showing their age in the same way as many other ships, and there's no real reason to replace them anytime soon.
The Aberdeen vessels are at their maximum size, they can't fit anything else into the harbour. What's rather needed is to replace the freight ferries with new ferries with additional passenger capacity (perhaps cabins-only) to take the strain off the passenger boats. The freight situation is getting desperate for islanders, and without a substantial increase in capacity, things are likely to deteriorate further.
One potential option could be to take on the Isle of Lewis, as she was designed to deal with the freight demands of the Ullapool-Stornoway run. She has a decent amount of capacity and would be available cheaply, and as she was designed to deal with The Minch, she shouldn't have any problems with Aberdeen-Lerwick. Northlink would still need a pair of dedicated freight vessels, but having the Isle of Lewis providing additional passenger capacity could work in the medium term.
One of the big problems CMAL has is that virtually every port is restricted in some way - this makes it very difficult to standardise the fleet, which would simplify maintenance and crew training.
I was involved in some discussions around the time of the Caledonian MacBrayne tender back in 2006, and one thing that stood out to me from a mile was the ramshackle nature of the CalMac network. Everything was more or less just thrown together rather than being planned with the long term future at hand, and while the CalMac staff themselves do an admirable job, it's very painfully obvious that the Scottish Office didn't prepare well for the RORO era at all. The sensible thing to do from the 70s onwards should have to rebuild the major ports so that they would be able to accept vessels across the fleet, while also having the possibility to accommodate the small vessels as and when needed.
Instead, CMAL have ended up with a complete mess. They are trying to correct that course with things like the new Brodick terminal, but it's still nowhere near enough to meet the needs of a modern ferry network.
I'd seen a few comments on social media questioning weather Caledonian Isles will ever make a return to service, do we know what the issues are with her?
There are certainly voices on Arran saying that they don't think she'll ever serve Arran again, and that the more likely scenario is for her to be sent somewhere that won't be so demanding at sea, such as the Mull run. My source "in the know" says that the plan internally is to move heaven and earth to get her running throughout the summer on the Arran route, but that there are serious questions over whether it even makes sense to risk Caledonian Isles in Ardrossan. Either way, with Ardrossan no longer being usable once Glen Rosa is launched, it might even make more sense to simply move to Troon now and accept that Ardrossan is finished as a CalMac port.
Having said that, Troon is also not really ideal as a port, as the train timetables are terrible in comparison to Ardrossan. It essentially puts on an hour for the Brodick-Glasgow route, due to the lengthy transfer time between ship to train.
If I was to travel over to Arran, would a foot passenger return allow me to go out via Ardrossan and back via Troon?
Return fares are 2x the single fare, so there's no reason to really buy a foot passenger return.