Mr.O
"Black Sheds, take a look at post #1, it has the links to the latest report. From memory, one of the pilots warns that they will soon be at 10000 feet, so he believed the altimeter and it was falling."
I propose that this statement was Made by PNF. It is an 'observation', and the PF was.......flying. F/O (RHS), was PF, he had duff (and unrecorded) data, this made him the one flying with displays that will not be known. Later, at dual ss input, the PF gives the ss to LHS (relief pilot). This is an intuitive notion, a "maybe you should try something" action?
RR_NDB
"Pitotīs failure (e.g. at cruise) are not sudden ("digital"). They start to be transformed, in altimeters for example, gradually. Thiells 727 (NW N274US) showed this gradual deterioration. I think that an encounter of a dangerous icing condition when cruising, is also gradual, technically allowing a warning (of this crucial info) before "law switching" by the Systems."
With no liquid water available, the assumption is entry of ice crystals? If so, the likelihood is they collected at the drain, and slowly plugged it. This increases the pressure sensing and increases ias.; the aperture then plugged, and stabilized the pressure, then the plugs melted, and the actual pressure became available? This happens over a period of time, as you say, and since in solid state, the water has slower Thermodynamic behaviour, lengthening the event?