Transactional sending and MRN field

I have a question regarding sending a single email to a contact and how to retrieve the bounces for such sending.
Currently we are using JobSubmitRequest XML API call for sending emails to a single contact and for each send we receive return value in the MRN field.
The problem is that while for bulk emails you can ask for status of multiple contacts using one MRN field, in transactional sending
We have a different MRN field per transaction and this means that we have per transaction to query on the bounces. Since bounces in emails can be synchronous or a-synchronous this can lead to multiple requests and if we send millions of transactional it is a problem.
Is there an API call where we can give multiple MRN values and get the results for multiple transactional sending in one API call?
Is there any other method which you can suggest for us to retrieve the bounces of transactional sending’s?
Is there a dedicated API calls for transactional sending and bounce retrieving that we are not aware of?
Is there a way for us to set an MRN value in sending’s and then query about it? (so we can use the same MRN value for multiple transactional sending’s).