Your quote left out "subject to a confidentiality agreement". Streamers are not obliged to make this information public, and I don't expect companies like Amazon to do so since they're extremely secretive about viewership numbers.
I.E. They do not have to release the numbers publicly, but they’re not really bound by NDA either.
I imagine most, if not all, will go the direct route so they can spin the numbers how they want.
No, the Guild would assemble an audit committee of likely three people or a third party auditor would be appointed. It absolutely wouldn't be shared with the entire membership. It would be extremely easy to find the source of a leak, and they'd be subject to ruinous penalties.
This is existing practice in a few areas.