Yes, our music is free to use in YouTube videos (even monetized), but you need to credit the artist in the video description.
If you don't want to credit the artist in the video description, then you have a two options: 1. You need to buy a license for each track that you use and then you can use it without any restrictions. 2. If you don't want to buy a license or credit the artist, you can still use our music in your videos, but we will put the copyright claim on your video. You don't need to worry about that, it does not cause any negative effect on your YouTube account standing. 
A 'copyright claim' does not cause your video to be blocked, and it does not cause any negative effect on your YouTube account standing. It is not the same as a 'copyright strike', which is a more serious issue. If you receive a copyright claim, you can add the attribution credits into your video description and contact us and claim will be released in 1-2 days.
You will need to buy a license for any project out of YouTube, and if you don't want to credit the artist into your video description on YouTube.
Yes. Once you have a license to use music, you can use that music as many times as you want, in as many projects as you want as long as the projects are covered by the usage terms of the license.