Office 365 Message Encryption questions

Few questions about email encryption 

1. If user has only Office 365 Business Premium + Azure Information Protection P1  license how does message encryption work in this case? Is there possibility to get "encrypt" button in desktop outlook or is it only in ProPlus version?

2. User has shared mailbox added to Outlook desktop app and user receives protected message to shared mailbox. How to open that message in Outlook if the message comes from other internal Office 365 user?

3. Is there any reason to send protected messages inside Office 365 tenant? Messages are already encrypted and protected?