Configuration for subfinder is saved at ~/.config/subfinder/config.json which can be copied between machines.
No. The majority of sources for subfinder don't require API keys, you just won't see as many results. Check the post installation instructions for which sources require API keys.
We're extremely open to pull requests, if you wish to have a feature feel free to develop it and push it to here for review and inclusion in the main project. If you're unable to code something, please raise it as an issue and if we think it has benefit to the community we'll look into developing it for you.
There are a number of subdomain tools, and we beleive you should try each of them to find the one that works well with how you like to approach subdomain discovery. We've developed subfinder to solve subdomain finding in a way that we felt worked best for the way we work, and was easily maintainable to allow us and others to add as many sources and features as possible.