Let’s say there was a perfect API present and WA allowed you to use a completely FOSS app witch didn’t share any data with FB. But still FB will know everything they can still access all the metadata like earlier. The only thing they can’t access is analytics data but that can be easily blocked by DNS. And as far as Add ID goes use graphene OS it protects against that by giving each app different ID. So even if FB collaborated with other services to collect data as they do they won’t be able tie all back to you.
This is a misconception. If the sender is outside proton mail the emails arrive in plain text. So it is possible for proton to read those emails. It is just that they pinky promise not read them and immediately encrypted them with your key. But if they wanted to they can read and moreover SMTP means your email has already traveled through multiple MX which could read your email but This is mostly not an issue since most email provider do encrypt with SSL of receiving MX but you might want to check few services use very very outdated softwares. But keep in mind SSL encryption is with Proton’s keys and by necessity they have to first decrypt the SSL encrypted email and then encrypt with your key