What we do is create mailboxes within our Office 365 instance. That way the mailboxes are branded with our company domain and a part of our address book. We instruct people to send emails to these mailboxes, and we have configured each of these mailboxes to redirect to ServiceNow on our email server. The Inbound Actions in ServiceNow see what mailbox the message (example: helpdesk@ourcompany.ca) was sent to and execute a specific Inbound Action for each one (example: what group to assign it to). 

This aligns best with your option 2, except there is no intervention required by ServiceNow itself. You will configure the mailboxes in your environment to redirect to companyname@service-now.com and your Inbound Action then handles what to do for each address that is redirecting to it.

We do not disclose our companyname@service-now.com email address to any users. We want to take advantage of our company branding and not force people to think about who they are emailing. Every email is to mailbox@ourcompany.ca.

What is initially confusing for most is "if you forward the message how does ServiceNow know where it came from?". That's why I use the term "redirect". Redirecting a mailbox allows the original message to be intact, forwarding means that we replace the "From" address similar to you manually forwarding a message. We set the redirect at the mailbox level by asking one of our email admins, but a user could create their own rule to do it. We found users confused redirect and forward too often, so we just do it for them 😉

I hope that helps! Any questions please let me know.