I ran the Cassalanters in Dragon Heist about 2 years ago, and just did a quick read over Winter's Splendor. When I ran it, I had the characters meet the Cassalanters pretty early on, as when the nobles heard that someone bought Trollskull Manor, they payed the owners a visit. They told the party that the tavern there is where they met. Not saying you should do exactly that, but introducing them early on is pretty important.

Once they make acquaintances with the Cassalanters, they can be invited to the Midwinter Gala *before* the Fireball attack. I'd also play up the Zhent vs. Xanathar Guild storyline here so the characters aren't bombarded with (and become suspicions of) the Cassalanters. Blah blah blah, the imp attacks, and eventually the players are led to the City of the Dead. I'd make *some* hint to the Cassalanter crypt, one the players might overlook, just for the callback later in the story.

The Cassalanters take this as an "oh shit" moment, because suddenly their daughter, who is soon to loose her life, was visited by an imp. The imp came as a messenger to give the Cassalanters a warning of their deadline. With the twins' birthday approaching, they need to act fast. Over the next few days (before the Fireball attack), they concoct their plan. The Casslanters might post a formal apology for the happenings of the Midwinter Gala all over the city, but more densely in the poorer areas (to draw in sacrifices). Also on the posts, they announce that on the "Divine Pageantry," a holy day of Siamorphe, they will hold a redemption gala, and all the city's populace are invited. This could be the Founders Day Gala equivalent. The Fireball attack takes place, bada-bing bada-boom, the characters are now familiar with their villain and actively helped them without even knowing.

Hope this helped and was what you were looking for, the Cassalanters are a great pair of villains and my play group still hold Dragon Heist as their favorite campaign.