At this time, the external tool option is exclusive of all other types. So if we assign a template which is pulled from Google Drive, the student can only submit a document which was created from that template, and pulled from Google Drive the first time the student clicked "Submit" for that assignment.
If the student has already created a document separately, that can't be submitted. Also, if the student wants to resubmit, it can only be that document which was originally created the first time "Submit" was clicked, (albeit modified in the intervening time).
Would it be possible to have the submission type be EITHER "external" tool OR "online," so that the student may submit a document that they created apart from the Google Drive template?