Hi Per-Magnus,
Generally, the items in the Deleted Items folder will be deleted from there after 30 days by default.
You are right that the retention feature is not available for end users. For your sub-goal, currently it is not feasible to achieve it. Thanks for your understanding.
To make items in the Deleted Items folder never be deleted automatically, as the admin, you can customize a retention tag, add it to a retention policy and then assign the policy to the users you want.
Here are the detailed steps:
a. Login to Office 365 Exchange Admin Center (http://outlook.office365.com/ecp).
b. Navigate to compliance management->retention tags->create a new tag as the picture shows:
c. Then go to retention policies->create a new policy->click + to add the tag created above.
d. Under recipients, click the user's mailbox->mailbox features->select this retention policy.
e. It will take some time for this change to take effect. Wait for the provision and then right click Deleted Items in the user's OWA->Assign policy->select the retention policy you customized.
Here is my result for your reference:
If you encounter any problem when trying these steps, please feel free to let us know and we will provide further assistance.
Best regards,
Chris Ni