Yes, believable, from all the payment methods available, Greenpeace would choose the most fucking inefficient one, that wastes 700 kWh for a single transaction, that's 100 households!
For a generic non personalized spam, IMHO it would be too expensive to generate and track millions of wallets. They could have placed a tracking pixel for much less (they didn't, the email is just plain text)
If then it's some targeted campaign, then yes, a dedicated BTC address makes sense as you said
In the past 24 hours a block contains in average only 3500 transactions. Then that block needs to be validated by many other nodes in following calculations.
This is why it's the most inefficient payment method, very slow (only 3500 transactions in ten minutes instead of few seconds), expensive for the user (transfer fees are high) and power hungry
Not only can people be pretty dumb sometimes, once the screenshot is on the Internet, who knows where it might get reposted, potentially without context.
You're not wrong, greenpeace is oil funded and has always been employed to redirect environmentalism away from anything that actually hurts the oil industry.
That seems high, though I guess if they're doing it in a state with high renewable energy, that's what they're using. It uses a crazy high amount of energy though.