If it’s a tiny bit of comfort, you probably don’t need a new derailleur, just a new cassette with the right spacing. The mech itself will likely work with different cable-pull, as long as you’re not drastically changing the number of sprockets on the cassette
It would have a massive effect. Transport (car) emissions are one of the larger - and growing - sources of emissions.
And we can’t hide behind “But the corporations…” because ultimately what they produce gets used by us.
So to answer your question: riding a bike when Global Capital wants you to keep buying cars and pumping oil into them is one of the best acts of defiance you can make