Yeah, I mean more that it's surprising Marissa would have even broken the partnership if she didn't already have a pretty good idea that she could line up a new, workable partnership.
That is typically the way pair splits work though. You split with your partner, with no guarantees that another partnership will materialize, never mind be better than your old one. Things can get ugly if you try to look for a new partner behind your partner's back. Even if you don't end up finding a new one, your current partner would still be left in the awkward position of feeling not wanted, and there might be trust issues and feelings of betrayal moving forward. So the best thing to do is to end the unhappy partnership first, which is what Marissa did. She and Simon were together 8 years, so she likely wanted to be as respectful as possible towards him in the split. And it has been said continuously that they didn't have a strong relationship, so that may have played a real part in Marissa wanting to end it.
I hope I'm wrong but I just don't see Leng and Shnapir as a particularly good match. He brings fabulous pairs skills to the table but he's not the most exciting performer. Castelli was and that's why they worked. Even when they had errors, her charisma saved the performance every single time. He needs a partner with star quality to show off and he's effective at doing that. With Leng and LeDuc, they were the rare pair where you watched him instead of her. It seems like Shnapir's team just went down the line of available girls from Nationals with semi consistent triples and she was the highest ranked and now we have a team of the 2 less exciting halves of 2 good pairs. I do get that the pressure was probably on to find someone fast before GP deadlines and she's agreed to go to Boston, which I'm sure was a big plus in her favor.
Interesting point. Leng seems reserved, not particularly expressive on the ice. The girl of the partnership is to be the "flower" to the guy's "stem", but I would consider Leng to be a bit of a "stem" like Shnapir. I can't see them having a particularly good rapport on the ice, but they didn't really have that with their previous partners either. They're different in terms of style. Height wise, I think they might look a bit better on the ice than Castelli & Shnapir (Leng is about an inch taller than Castelli), but it's tough to say how they would mesh in terms of presentation or technically (I do think Shnapir is probably a stronger pair skater overall than LeDuc). I agree that these skaters probably rush sometimes to try to get something together fast and take whatever's available. It must be difficult to know when it's better to wait a while and see who's available later on. But usually there aren't too many options.