How to migrate email database from Outlook for Mac 2015 to Windows Outlook?
You are absolutely right that there is no way back to switch from Outlook mac to Windows Manually as outlook mac exports its database as .olm files which cannot be imported into Windows Outlook. 

If your email id is Imap enabled then you can directly login to windows outlook and within few minutes your emails will start downloading.

There are many applications available that help in transferring the outlook mac data to windows outlook. You can go for these only if your email database is large. Here is one I found that had some decent features: http://www.olmtopstconverterultimate.com/

Please note that these applications are paid. Give the trial version a thorough run g=before purchasing the complete license.

Thanks. I hope this helps.