Thanks. You appear to be correct about being able to repeat DIAD. The duration, however, is not quite that clear cut.
Total Rewards customer service informed me that if you achieve DIAD March-Nov, it expires the next March (so <1 yr duration, whereas the traditional Diamond tier lasts that + 1 year). If you achieve it Dec-Feb, it lasts past the next March and expires on the following March.
If I understand their explanation correctly, if you achieve it in December, and only December, it has the exact same duration as if you had achieved it with 11,000 tier points.
I will be attempting this in a couple weeks, just for giggles.