No. Since the app is not installed by the host/account of the meeting, you cannot get their data.
Yes you are free to make app updates. They will be reviewed by our App Marketplace team and then your app will be updated. Usually it does not take too long for update requests, a couple days at the most.
The app would remain unaffected, but in order to access certain features the app would need to be on a pro account. If your app is transferred to another users account, you may have to have us update the backend to allow you to get access to it, but the app itself will still work.
You can ask the app testers in your submission notes to provide their email.
Here is a common flow for only allowing organizations that signed up for your services to install your app:
