G'day Michael and thanks for applying your head to this problem.
On 1. I think there's a reasonable chance of this and I might contact the tank manufacturer with the symptoms to see what they think.
On 2. I agree and I think that if the thermostat had been failing to cut off at the set temp. there would probably have been boiling and pressure release through the valve but there were no signs of that, so maybe that supports 1.
On 3. I've replaced it, though I've yet to run a test since then.
On 4. The power has now been off for more than a week and the solar side of things has been supplying the heat which seems to be as hot as it ever was, so I'm thinking that this indicates there isn't a significant leak because, particularly with solar heating only, there should be a drop in temperature. Put another way, under circumstances where the element would have been using 12kWh per night its absence hasn't caused any noticeable change in water temp ... maybe that leads back to 1 again but have to keep an open mind
On 5 Aaagh. Origin reckon that if they come out to test the meter and it isn't at fault, there's a $300 charge. I think it can be ruled out though as there has been a meter change-over recently and it's telling the same story. Initially I thought it might be the meter till I went back and collated the figures showing a gradual rise over 12 months.
On 6 The solar is excelling and doing a good job of showing up the mains system, at least now, in the middle of summer.
There is a slight chance that this is a tech. problem as there's a small computer board managing the solar pump [and possibly overseeing the thermostats].
I'm surprised that the electrician didn't take on the symptoms and come up with an instant diagnosis. With so many of these thanks around, this has to be a common thing.
I'll tell you how it goes .....
Primary system: .8KW Trina panels; Plasmatronics PL40; 1,000Ah VRLA 12X2V battery bank; 1.7KW Selectronic inverter
Veteran system, semi retired: 6 X 55W Solarex 12V panels; Fangpusun CC; 6 X 2V, 35 year old 550Ah lead acid flooded cell batteries.