Unfortunately none of us can say for sure what might happen with Cotopaxi. I believe the experts expect the volcano to be settled down by the end of the year, but of course no one can say for certain. So far Cotopaxi has not been interfering with international flights in and out of Quito so I wouldn’t worry too much about your flight right now.
As for El Nino we are expected to have heavier than normal rains this year and the coast is likely to get the worst of it. I would leave an option open to bypass the coast if flooding is a problem at the time you’re here and spend more time in the Sierras.
As you said no one can predict the future, so if possible I would try to leave my trip as flexible as possible just in case. Though in all likelihood your plans will work out just fine as is.
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