I would make sure they look at anywhere where water could be getting in or leaking. Any chance the plenum below the windscreen is blocked with leaves and crud so water isn't draining? Have you seen heavy condensation on the inside of the windscreen at all lately? Also look for signs of water staining in the luggage compartment left where the radio / amp is.
The sort of random and intermittent weird behaviour you describe is often caused by water ingress and penetration into the connectors in electronic systems made up of separate modules joined by a control network. Intermittent shorts on the CAN bus or the like.
Important note: do you have the Air Con on all the time? There was a Tech Service Bulletin on an issue with the evaporator drain pipe where it is is badly fitted causing moisture to get into the cabin behind the dash and into the footwell. Your CAN gateway is in there to the left of the front centre console and your 5F information module and harnesses are in there (not to mention a few other control modules) which may explain why some functions from some separate modules located elsewhere on the car are disappearing and appearing again - like brake assist, and the convenience module functions. If moisture is getting in it will condense on any smooth hard surface like the metal dash mounting as the temp drops overnight. Once it forms heavy droplets it'll flow along any continuous route like pipes and cables and end up pooling and seeping into the gaps between plugs and sockets.
Deffo worth checking properly. Look for slight signs of rusting, corrosion, or patching on bare metal edges and corners on the 5F box. Easy to pop it out with couple of radio keys and have a look before you go to the dealer.