1. I would make opt-out the default. Then offer an opt-in for those that want it. What about the granularity of opt-in on specific articles? Leaving Medium - probably not - it seems like you've taken the right approach with this.

2. The search engine credit should be required. However, it isn't sufficient because we should be compensated for our work.

The problem with credit is in human nature - people are in a hurry doing their work and want a quick answer. That means that once they get their answer, they have no incentive to read your article other than having extra time and curiosity. The fact that the answer is attributed to multiple sources decreases the probability that the user will visit your article, especially if it's toward the end of a list of sources.

Thanks for doing this.