Of course you can!

Look via the interrail website, click on make reservations. If you have purchased a pass, it will appear on the website when making reservations.

You activate your pass the moment you leave.

Alternatively, make reservations at a ticket office or on the operator's website.

If I made my seat reservation for tickets online through Eurail website and received/printed out my e-tickets (each has a QR code). Since my tickets are printed do I still need to activate everyone’s Pass on the App?  

Yes, you always need to activate passes in the app because only then your Interrail will be valid. Remember that your Interrail is your ticket.

Reservations are separate and only needed for some trains; you can travel on many trains with just your Interrail pass.