Changing domain names and replying to sender to notify about the change

Our client recently acquired another company and we're now in the process of adding them to Office 365. Normally this would be straight forward: Add the domain, verify, setup Exchange/Lync, add users, etc.

In this case however there is a domain name change involved and our client would like the following:

1. Add all users to the new domain
2. Have emails sent to @olddomain.com forwarded to @newdomain
3. When email is sent to me@olddomain.com a reply should be sent to the sender to notify them that the domain is changing.

So my thinking is the following:

1. Add both olddomain.com and newdomain.com to Office 365 and do the verification steps
   Question: For the olddomain do you just need to verify the domain? Not the third step of defining how the domain will be used i.e. Exchange Lync?
2. Create all the users using the newdomain.com domain
3. Add aliases/proxy addresses for each @newdomain.com user adding their @olddomain.com equivalent. I assume this means they'll receive email from both domains.
   Question: I noticed in the new O365 Exchange interface when adding an alias it doesn't have a drop down list allowing you to choose from an existing list of domains in O365. Am I right to assume that as long as the OLD domain is added to O365 (DNS MX records, etc.) that mail will be routed correctly?
4. Setup replies to senders sending mail to @olddomain.com
   Question: Here I'm clueless. How would I go about doing this on the server?

Thanks,

Jacques