This is just my observation on how questions can be added, deleted, reordered.
First, if questions on a page have been attempted, you can NOT add or remove questions. In the past, I have duplicated the activity, which then allowed me to change questions BECAUSE no questions were duplicated!
If questions have not been attempted, you can click on the, Add new question, button. From the page of questions that appears, you can select up to three questions. They seem to be added to the page in alphabetical order based on the question name.
If questions have not been attempted, you can change the question name to get the order you want.
If questions have not been attempted, you can change the selected questions to add or remove questions, so long as you select three or less.