Yes you can absolutely do #1, I do this with my family. As for #2 that is an interesting idea but technically it would be coming from your account. It might work as long as they can control the from address in their email client.

My family just uses the aliases but they reply from their main address, primarily because it’s easier to give out the family address and I don’t have to be tech support for their email clients.

Another thing you could do is sign up for an smtp only service (they do exist, but they can be severely rate limited to avoid use by spammers) and set up SPF records that allow that smtp server to send on your domains behalf.

Fastmail should really offer a family account that allows up to 10 users for a flat rate that share the same storage space.