I think plastic straws are likely to be your best bet, but here are some alternatives. In each case I'm aiming for popular causes (not just individual laws or the like) where your readers should be able to see why people thought it was a good idea, even if ultimately unsuccessful:

• Prohibition
• Liberia
• Opposition to GMOs
• Opposition to Nuclear Power
• US anti-war activism prior to Pearl Harbour
• Western pro-Soviet activism before their atrocities were well known.

I like the example of the anti-overpopulation movement of the 1960s and 70s. It involved good intentions, but its predictions and fears (e.g. widespread famines) were completely unfounded from today's perspective. It also produced some very unfortunate policies in developing countries: 

"Millions of people were sterilized, often coercively, sometimes illegally, frequently in unsafe conditions, in Mexico, Bolivia, Peru, Indonesia and Bangladesh." This article seems like a good starting point.

Opposing the Green Revolution in Africa here:

"Borlaug's initial efforts in a few African nations have yielded the same
rapid increases in food production as did his initial efforts on the 
Indian subcontinent in the 1960s. Nevertheless, Western environmental 
groups have campaigned against introducing high-yield farming techniques to Africa, and have persuaded image-sensitive organizations such as the Ford Foundation and the World Bank to steer clear of Borlaug."

The Green Revolution wasn't even GMOs - just dwarf crop varieties, fertilizers, and pesticides. Not only did opposition to this mean many more people dying of malnutrition, but also more rainforest cut down, so more loss of biodiversity and climate change. 

Maybe ALS, as exemplified by the ALS Ice Bucket Challenge? It is a problem, but probably didn't need an ice bucket challenge to focus on it, vs. a malaria ice bucket challenge for example.

Just linking two relevant resources - I assume you're familiar with the 2nd one:

What are the "PlayPumps" of Climate Change?

Looking for more 'PlayPumps' like examples

The first one might help you generate some ideas on sub-causes of climate change that are too much hype, such as the hype around lessening usage of straws/plastic bags in the West.

Not sure where this falls on the "cause vs. intervention" spectrum, but protests against nuclear energy are a clear example of people doing something net-negative according to their own values (by reducing the amount of safe, clean energy that the world can produce).

I'm confused by the fact that you want to show "prioritization between causes" but also that you want to show effective and ineffective examples of people working in a single cause area. If you're just looking for cause areas where some ideas were overhyped and effective ideas didn't get enough support, there are a lot of examples out there. A few that I had time to look up:

• U.S. education — the Gates Foundation has a few notable failures, and there are also many wasteful initiatives funded by governments
• Job/skills training programs in the developing world (and maybe the developed world?)
• Wildlife rehabilitation

Maybe some historic examples could be something where readers can look back and all agree that it was a cause where focusing on it made the world worse? Some random ideas:

• witch hunts
• appeasing vengeful gods with human sacrifice (ofc you might want to tread carefully around religious topics)
• the Shoa is a very horrific example of an anti-semitic ‚cause‘ (maybe still too emotional and politically charged, but maybe another genocidal movement could be used as a thing depressingly many people got behind)
• homosexual conversion movements (maybe also hits too close to home for some readers)
• „female sexual purity“ as a cause that led to norms of female genital mutilation and honor killings?
• some sects that predicted the apocalypse and did a lot of harm to their members with their preparations?

Great project. Really looking forward to it.

One example that came to mind is this old blog post by GiveWell comparing "saving a child’s life in Africa to that of helping improve a child’s education in the New York City": https://blog.givewell.org/2007/12/19/all-causes-are-not-created-equal/

There are a lot of good ideas in the other comments already!

To add one more: Fighting Genetically Modified Organisms (plants specifically). There are of course valid concerns about the technology. However, most of the concerns that I hear (and believed for a long time) are motivated by hearsay and by religion. Many Christian friends share a feeling that we shouldn't "pretend to be God" and interfere with his creation. On the other hand, many people lack knowledge about the benefits of GMOs.

The podcast episode that radically changed my thinking about GMOs is https://erklaermir.simplecast.com/episodes/167 (in German unfortunately, but highly recommended.)

By the way: you've probably heard about Bruce Wydick's book "The Shrewed Samaritan"? Its goals sound similar to those of your book.

Cause Area A: Blindness in the United States

Cause Area B: Blindness in the developing world

From Peter Singer's TED Talk:

Take, for example, providing a guide dog for a blind person. That's a good thing to do, right? Well, right, it is a good thing to do, but you have to think what else you could do with the resources. It costs about 40,000 dollars to train a guide dog and train the recipient so that the guide dog can be an effective help to a blind person. It costs somewhere between 20 and 50 dollars to cure a blind person in a developing country if they have trachoma. So you do the sums, and you get something like that. You could provide one guide dog for one blind American, or you could cure between 400 and 2,000 people of blindness. I think it's clear what's the better thing to do.

Not sure if it fits what you are looking for, but colonialism? A somewhat underappreciated aspect of it was that the belief that spreading Western civilization would make indigenous people better off, see e.g. White man's burden. Also, the Western powers were obviously very effective at colonizing. On the other hand, it somewhat lacks the part of the "play pump" example where everybody agrees that the responsible people had good intentions. Maybe it could be adapted to tales of Christian proselytizing in Africa, which would be relevant for your audience.