Do you know if it is possible to connect one alias to multiple mailboxes?
I don't know if Mailbox does this but technically speaking it should definitely be possible, as long as the mailboxes are on the same domain. (If you cross domains it's not aliasing anymore, it's forwarding.)
I know Migadu has this feature and it works as intended.
Speaking of Migadu it's another good option but they only do email, nothing else (no calendar, no drive etc.) They have an interesting pricing model where the account owner pays for a certain tier of storage and of emails sent/received per day, but inside that account you can have as many mailboxes, logins, domains and aliases as you want. It can become cost-effective very fast, as low as 2-3 users.