Suggestion for Solution #3 - For Follow Up Flag How about creating a To Do entry with a link back to the original SMS or possibly copying the information over to keep as self contained in case the original SMS is deleted.
Solutions 2 & 3 could be combined(I think should be) and still retain the same goal in mind? Send to Calendar goal is rather obvious. Follow up, would you not want to make some sort of calendar entry or be able to setup a Note / Alarm? Calendar would be a great focus point, and should have SMS/MMS, Email, Notes, Alarm and Contact integration. While in an MMS/SMS or E-mail, I click drop down menu and have additional options to "Create Event" "Create Alarm" & "Files" - Files can be Notes, PDFs, audio, etc. Add an MMS Contact direct to Calendar even to a "With Whom" field or maybe into the Description, but make the Contact name interactive on event page so when clicked it goes to their info or when long pressed, menu pops up to call, find location, etc.