1) Solar heater has no electricity to it. Why need electrical Earth?

2) Basically it is sufficient but Earth detection that trigger ELCB will only be triggered when electricity leak is detected in Earth circuit. If current leak through water to your body and to house floor, then it will not trigger the ELCB. This is where RCD is taking charge. However, logically, the heater element leak to water will eventually leak to copper heater housing thus will trigger ELCB. Anyway, having both will ensure fail safe operation.

3) Basically answer is like #2. Yes, no problem being served my existing 30mA I believe.

So basically like all other answer. Once there is current leak into water, the current will be conducted through copper and Earth wiring.

Yes, lightning is another story. No matter what, I think you would want lightning to strike a lightning arrestor than your heater because that Earth wire is not meant for lightning. The wire will melt on spot if direct hit.

Lightning is attracted to static charge, not the electricity. So, it is the whole house charge , not just the heater there. This is what I know.