(1) I'm not sure why the instructor would be taking the assignment in the first place, but you should be able to authorize additional attempts just as you would for students. If the instructor tries to take a test and has already exceed the maximum number of attempts, they can click the "Request exception" button and use their own username and password to authorize another attempt. Are you seeing something different (e.g. no "Request exception" button, an error message when the instructor clicks the button, etc.)?
(2) Questions can now be shared across all classes in the system with the Question Repository. To prevent an instructor from accidentally overwriting the content of another instructor, uploading two .qu files with the same name (e.g. questions.qu) will not overwrite each other. The easiest way to correct an error in a question is to locate the question in the Question Repository and select "edit" button. You can then make the corrections in the Question Editor.
(3) It's possible that incorrectly editing the source could cause problems loading the question. If you'd like you can upload the .qu file or email it to Maplesoft Technical Support and we can see if we can track down the problem. Again the recommended way of editing a question is through the Question Editor.
(4) Currently you need to click the + or - buttons to change the value of a question. I will submit a feature request to see if this field can be changed in the future so that it accepts numbers from the keyboard.
It isn't clear what problems you're running into editing a "live" question using the Question Editor, but again if you could send us an example (through Mapleprimes, to Maplesoft Technical Support, or through your reseller) we can certainly have a look.