I have previously rodded and cleaned the corner drains and confirm that are all functioning properly.
Remove the glass section by unscrewing the sliding trim from the inside front and sliding it back, which then exposes the six side guide screws for the actual glass unit. The glass cartridge can then be lifted out, exposing the mechanism in the main cassette.
The way tilt and slide sunroofs work is that the exterior seal is really a wind reduction seal, water is allowed to drain down into the main cassette and be carried away by the corner drains.
Due to the rear edge having to accommodate the sliding glass cartridge it has a floating gutter that slides with the cartridge, which catches dropping water and channels it away to the sides of the main cassette.
The issue I have at present is the water from the rear edge of the glass is falling down onto the sun-blind, then onto the front seats and into the headlining/lights console, especially under braking.
The internal floating gutter needs to be clear of debris and properly shaped/crowned, to allow water to readily drain away. What I believe was happening was the gutter had sagged/been pressed down and was filling with water and not draining. On moving away, then braking, the water then sloshed over the front of the gutter and into the car. It's possible the sun-blind was also filling with water when the gutter over-flowed as the car was standing.
I thought the glass to cartridge frame sealant might also be leaking, but having tested again it seems sound, with no apparent leakage, or glass movement. I have had one glass panel come unbonded, previously.
The gutter was essentially clean, but I flushed it completely clear and reformed it to ensure it drained at the edges and has a good centre crown.
I tested by squirting water from a bottle and the gutter appeared to drain quickly and fully, so hopefully this is resolved.
At some stage I will remove the headlining and sunroof cassette to deal with the seized mechanism.
Yet another vote here for the sunroof appreciation society...