I would translate のこと as ‘regarding’ or ‘about’. It shouldn’t really change the meaning but is just a slightly more indirect way of saying ‘the weather will be x’. Maybe you’ve also heard of あなたのことが好き(すき)です as a way to confess your feelings. The meaning should be the same as あなたが好きです but it might be perceived as more indirect and polite I guess.
While there seems to be a more specific explanation in this case, I believe it’s fairly common that for onomatopoeic or mimetic words with repeating syllables the last one can be replaced by り、ん or something else. E.g. ぺたぺた、ぺたり、ぺたん、ぺたんこ. So your result is close enough I’d say.
Wiktionary says it’s the other way around (until originates from til) and both til and till are correct: https://en.m.wiktionary.org/wiki/till#Etymology_1
Technically speaking, pretty much all of our power is solar power with extra steps.