I am reading these post because i am having similar issue; and i do understand what is happening.
I mail to our homeowner association of about 150 people and i really need to know if the got the memo or important papers.
Would it help if i add a link or something (open to suggestions) requesting that they click after they read the mail so it would force it to show opened?
In the case of graphics not being enabled having the reader click on a link does then count as an open.
That's actually a very good work around. Make it an obvious action ( with graphics disabled) for the reader to take. Perhaps placing the "please accept terms" reminder at the top and the actual link at the bottom which may give you more of a comfort feeling regading that they read it.