1) I would like to transfer my license to another development environment (new hosting provider).
Given that your account only provides for a single site key, you will need to delete the existing one and register your new development evironment with the new URL.
2) Later, when I will be ready to go in production, I will then have to transfer the license again to a new production environment
Similar to #1 above you will need to delete the current one and create a new key for the Production environment
QUESTION 2)
Once in production, is it possible to use licence in development (Test-Host) at the same time than in production (Prod-Host-B)
No this won't be possible to do as your account only allows for 1 site key and this key is tied to the URL of the website. You won't need a site key per say as you can still use the plugins normally. The only issue is that you will need to perform the updates manually on the site that isn't registered.
