Hello there!
To start with: this is a big project, but I loved it. I have been working with Google Apps for the past year, and I can definitely see the appeal to use them as basis for your system.
I do however, have to point out a few limitations. For one the graphs in the doc/docx files can not be made editable, as they will not be generated as graph objects, but as pictures. I don't think there is a way to make them editable.
For the purpose of identification, the e-mail address and a passphrase should work just fine. However, I will mention here that the security with sending the passphrase in the email will be not very high. Also you will have to walk me through the "opt-in for my mailing list, kept completely separately...", as would leave us only with some ID, while the reminder system will need the e-mail.
Your requirement of variable font is implementable, although it will take a lot of repeated programming. We'll look at the best way to make that happen.
Also whenever you are thinking of interaction with feedback (tell us who you are, we will tell you your previous results), basically it will mean that one form will have to be submitted and the result will be sent by e-mail, it won't be possible to have it as a next screen in the same form, technology does not allow that.
As for doc/docx/pdf, all are possible (graphs not editable though).
Reminder routine can be scheduled to run through the spreadsheet as you said and send e-mail, whenever it hits the date.