Regarding the $40,000 DAI, they were no obligation to leave that money in an escrow.
You are right in saying they were under no obligation, but my guess is, they used that as a reason to gain more trust, it was an important eliminate to compete the circle.
If that was part of the manipulation they did they did a very good job to win the trust of very large parts of the community.
If anything would make me doubt that this was a scam plan from the get-go would be something like what @hugeblack mentioned above, a single person running this whole thing, and something bad happened to him, and it's game over. Again (nothing better than being a scam as far as everyone who lost funds is concerned), so ya, it's not the money they spent that makes me think this might have at least 5% of being a failure rather than a scam, it's the efforts and the intelligence of the person that could have otherwise been used into making a good mixer that would last for years and make a lot of profit.
Keeping the single person operation possibility aside, the earlier post showing a similarity between Tornado Cash and Whirlwind could also have some relevance but we do not know for sure. What I do remember is that the Whirlwind forum representative was intelligent and when reading his posts you knew he was an articulate developer. It has been a huge disappointment to see how this came to an end after such a promising start.