Electricity for light let 1V go
Is it normal for these to have a low voltage even when switch is off?

Yes. The internal resistance of your multimeter is very high (mega ohms), so even a tiny leakage current (fractions of microamps) through the capacitance that exists between wires running in the same conduit will be enough to read something on the multimeter.

Could that be what causes my RCD to break?

Nope, by 5-6 orders of magnitude.
I don't think the culprit is that wire nut. 300mA is a lot of current. Even if the wire nut was submerged in a bucket, I'm not sure it would pass that amount. I mean, it's pretty hard to trip a 300mA RCD unless a wire gets loose and you get an actual short.
The other wire nut only has two holes, so unless they're Live and Earth, if water gets on it it won't trip the RCD.
I'd suspect a junction box somewhere, or maybe a wall socket, or a switch, or something that gets really wet. Or it's not the rain, it's the wind instead that rattles some loose wires. Or the rain causes the mice to go back home, and as they get bored they chew on your wires. Or the insulation on some wires has been damaged, and they touch something that gets earthed much better when it rains than when it doesn't, like a metal gutter.
PS: you should really install a 30mA RCD, they're lifesavers.