What I found interesting about the wedding dress event was Liam's unprofessional behavior. Of course Cathy had a dream figure but should be constantly surrounded by women like that at work. He's there as a consultant and he's acting like an immature teenager. The woman wants to get married and he takes the opportunity to check her out. Maybe a coincidence, maybe L&P is again a reference to the character of Liam.I don't think we can "trust" him with this, the bride event was not scheduled either, not to mention coach Sophia. I think as he develops the story he sees new paths or new events that could fit well the story. The problem is, obviously, that it is already complex as it is, adding more branching will make it longer to develop etc etc all the things we know... But to be fair he said "subject to changes" on this one...
And for the concert (sorry i edited while you were probably answering) yes probably it will be before the other event as you said. But I don't know if they will split though, after all Sophia didn't say anything when Cathy proposed.
Also if this is to (start to) set up the swinging path I guess it will be in the GW route (but maybe KillJoys can remind us better)
What happened during the dress event seems to have a different connotation whether Sophia left the company or not. It could then be again another branching event whose evolution is different if you did or not the 3rd task. Like, no 3rd task means swinging, 3rd task could mean that something different happens. The immediate logic would suggest something goes on between Liam and Cathy but we also know that L&P said that Liam is not cheating. But again, maybe he changed that again, in the FW route leaving for the task you set something in motion between Liam and Cathy that later on will give the finishing stroke to the marriage.
As for the invitation from Cathy, I don't believe in the beginning of a swinger route. Again: They want to get married (!) And suggest swinging to a couple of friends? I think the invitation is used to propose to Sophia and Liam how to organize the bachelorette party. Sophia for Cathy and Liam for Andrew. On both the Good-Wife Route and the Filty Route, these parties could start separately but maybe end together in a club. Then maybe swinging would be possible with a lot of alcohol. The parties could just as well stay separate and Sophia and Cathy have fun with the strippers.