@CaseyGrant -
Well I have confused myself with this question. Now I am seeing the same thing you are once a student submits a quiz, the assignment is marked complete and can't be changed in speed grader. I tried to find an explanation of the complete/incomplete grading in the instructor manual and failed.
from what I am seeing now it seems that whenever an answer is supplied to a quiz, then it gets marked complete. My guess is that it has to do with the grade obtained. You could try playing around with possible scores for the quiz and questions (by the way this is all in New Quizzes). I thought I could do incomplete/complete in classic quizzes but evidently not.
It is strange that speed grader is not allowing you to override (or just enter) a grade once the quiz has been taken.
I am unable to help any further - though I may play around with it some more. It would be nice if someone from Canvas would see this and be able to chime in with how the complete/incomplete option works and how it interacts with speed grader.
I did a search on the community, and the following may help:
https://community.canvaslms.com/t5/New-Quizzes-Users/New-Quizzes-survey-workaround-and-complete-inco...