There can be cases when student forgets to click submit button - is there any possibility as a teacher to click submit for him or in some cases even to cancel his quiz if he turn off camera and keeps working in quiz.
Hello Beta,
... is there any possibility as a teacher to click submit for him...?
I do not think so. But, in the quiz settings, if you set a "Close the quiz" time and/or a "Time limit", and if you set "When time expires" to "Open attempts are submitted automatically",
the quizzes will be submitted automatically even if students forget to click the "Finish attempt" / "Submit all and finish" buttons.
...to cancel his quiz if he turn off camera and keeps working in quiz?
You can always cancel a quiz after it is finished. If you want to cancel the quiz while it is running, set a "User override" for the student with the "Cloze the quiz" time enabled and set to a previous time.