How should the number and length of breaks be managed for students who receive extra time as an accommodation?

Breaks are included in the additional time students receive. Students who require breaks cannot be asked to limit the number of breaks they need to take as they might be managing pain and/or symptoms that are unpredictable.   
The solution is to allow as many breaks as the accommodated time allows. If the number and/or length of breaks is flagged by proctor track, discount it.  

An alternative is to provide students with an alternate format of assessment like a take-home test or alternate assignment.

Access to scrap paper (e.g. for writing out calculations)

This is a strategy that we teach students who have working memory and retrieval issues.  Students with anxiety disorders might also find it helpful to write down concepts they are concerned they will forget.  In cases where students ask for access to blank paper, they can hold the blank paper up to the camera at the start of the test/exam and then show it at the end. Students could also take a picture of the scrap paper and upload it as a part of their test/exam.

The ability to scroll back and answer earlier questions.

Students with memory issues often need to read through a test/exam to start priming their memory.  

If this is not permissible, there might be the need to find an alternate format of assessment for the student.  
Questions divided into time-limited sets.

​The test/exam could be administered in chunks, with additional time applied to each set of questions.  This would help to ensure that a student doesn’t miss a section of the test/exam because they needed to take a break.

If this is not permissible, there might be the need to find an alternate format of assessment.