This seems to be limited to options with checkboxes, is it possible to construct a similar field using radio buttons or another available option which does not contain the All 

There are ways to construct custom forms so that they do not have an All.  You would need a system admin to edit the custom forms.  Whoever built the custom form specifically created All as a reponse, the corrective action would be to edit the field in the custom form to remove the All answer.

Have you tried the proof approvals widget on the new home screen?

I have never heard of any system admin limitations and our environment has more than 2 global admins.  If you do not have the ability to designate someone as the system admin I would question whether you are global admin of your Workfront instance.

You will not be able to read any text on raster image formats within the proofing viewer, that's not just a limitation with Workfront but it speaks to the nature of those filetypes.For text specifically you might have better luck using RTF instead of TXT.

I can upload PNG files and mark them up, can you elaborate?

This has happened to me many times and it's usually the result of the email address being added to a proof at some point.  When this happens the email address in the proof is added to the Workfront user list not as a name but as the email address.  Simply search the users for that email and update a...

Would it be simpler to somehow adjust your workflow to upload individual proofs so that these individual documents could be approved?If you upload a document with 50 pages and you want to approve 2 of them, what is Workfront supposed to do with that information?  From Workfront's perspective, the en...

When you have a dropdown field in a custom form you can choose to have one of the results be the default selection by clicking the corresponding gear icon and choosing Select by Default