If you swap between live and neutral the water heater can still work. But if you swap the earth every time you switch it on your elcb will trip, that provided the elcb functioning. Hope your problem is only wrong wires. Look to me maybe leakage, did u change the switch because the original switch melt?

First check if you install correctly. If yes and confirm not that problem. Maybe is your wiring or the new switch got problem.

You always use max heat on your water heater? Seldom melt unless always set to max.

All Earth need to be connect because some are loop to another circuit. You can trim some copper to fit all in. Earth don't carry current it will trip if have current.

Not sure if that is the problem but if that fix that mean you have earth leakage and your elcb not functioning. If function your elcb will trip.

Use test pen.

Both yellow is live? Or one of the yellow is live? If one yellow is live another not, than someone wired wrongly.

If both yellow are live, maybe the boiler wired to a wrong set of wires

That mean the person who install for you just make it permanent on earlier.

Is the new switch working accordingly?

Not sure how the heater are wired, cannot give you advice without knowing how is wired. Better ask someone with electrician knowledge to be on site to help.

I fix many different brands of heater switch, and model. All thr red light will off wheb os off. Some model the wiring for the indicator are external, and you need to tap to the correct side, if not the indicator will permanent on.