According to XEP-0313, it is not possible to remove an item from the middle of the archive while PubSub-MAM does that.

A server MAY impose limits on the size of an individual archive. For example a server might begin to discard old messages once the archive reaches a certain size, or only keep messages until they reach a certain age. Any such deleted messages MUST be the oldest in the archive, i.e. it is not permitted to create gaps or "holes" in the archive. The UIDs of deleted messages MUST NOT be reused for new messages.

However a server that wishes to remove messages from the middle of an archive, e.g. to remove accidentally transmitted sensitive information may omit the stanza from inside the element or replace the message with an appropriate placeholder when transmitting the result in response to a query.

This needs to be fixed before we can work on MIX.