If Koresh knew something about ME terrorists (or anything else), why not take him out directly? Or was somebody trying to get two birds with one stone?
Similarly, what changed in the middle of April that the decision to go in with the tanks was made? The only thing that comes to mind is the Davidians were almost out of water and would have to come out.
I believe Waco was "taken down" in the manner that it was because they had no other choice - i.e. Koresh and a select few had WMDs and will to use them. There are many reasons for this, one of which you noted - here's another:
St. Louis Post-Dispatch 1/29/2000 Terry Ganey "..... Most chilling of all, Sgt. Barry reports: "Their operators had penetrated the building on several occasions, and on one occasion, late April 17 or early on the 18th, they saw Koresh within six feet of them. They radioed back to the Tactical Operations Center for permission to grab him, and within minutes the word came back from the Justice Department, 'No, we already have a plan in place,' that being what happened on April 19." ..." The joint chiefs were contacted on March 2nd with a document marked "secret specat." Six days later, top secret robots were sent to Waco. They already had the complex bugged for voice, so I suspect the top secret robots had a different function, something relating to detecting WMDs. As you noted, they began flash-banging the people who were trying to leave, put up concertina wire and began monitoring the water.
The April 14th second briefing with Reno (who has a degree in Chemisty) included Dr Harry Salem chief scientist for life sciences at the U.S. Army Chemical Biological Defense Command, Col Boykin of Delta Force and Gen. Schoomarker, who had once headed the Army's secret anti-terrorist unit Delta Force and was later at the U.S. Special Forces Command at Macdill Air Force Base in Florida.