Not a lawyer, but I do work with licensed content: In this case you would probable need to pay the licensing fee before turning on ads for any video using assets from this library.

But what if you do it by accident, forgetting that you’d used a licensed asset in this long-published video? Tough cookies.

If you’re going to publish a mix of commercial and non-commercial content, you’d best be using some sort of tagging system to keep track of which videos use assets from which libraries. Because if you mess up, it’s on you.

We frequently run into this issue when a vendor gets bought by another company who doesn’t want to renew their contract with us: At that point we have to use our tagging system to find any content using assets from that library and either pull it or find replacements from somewhere else.

> with an advertising revenue of £0.10 is considered commercial and a payment?

Yes. Ignorance not excuse. There are quite a few cases where people go on GIS and find some random image, and then stock image crawling bots encounter it and extract payment with penalties.

Even if they are not monetised by the uploader, the video will still be monetised by Google, so this is still commercial use, even if they only earn 0.0001c

YouTube is a commercial website, whether or not they give you a cut of their profits for your content is irrelevant. To upload videos to YouTube, you must assign them a license for their commercial use. You have no right to make that assignment of license if you do not hold the right yourself.

Yes