Hi mtraveller, and welcome to Flyertalk.

You will find that Hyatt will endorse your profile as Diamond once you hit the 25 stay mark, and your new card will arrive within 21 days.
Your trip on the 20th will be as a Diamond and whilst you will not have your Diamond card in your hand, your profile that resevations punch up, will show it. This should not stop you from telling them, or anyone else that's listening that "You are a Diamond".
The trouble with being a diamond is that you are no longer interested in what promos the plats are getting, in fact, you are not interested in anything other than what Hyatt does for it's Diamonds, so I can't help you with your bonus question.
Ensure that you read up on what diamond status gives you, understand the idea of booking base and receiving club, the tier bonuses and keep an eye on the hotel GP bonuses. Congratulation and regards bkkrop