Fortunately Casino Fatale is a big enough game that having two people arrive late shouldn't cause any problems. (It would be different in a 6-player game...)
The first thing is: when you read out the 'Welcome to Casino Fatale' announcement, add that those two characters are delayed on business and will be joining the charity event at (estimate time that you expect them). That will make sure that all the other players know that there's no point seekign out either of those two, or maing plans concerning them, in the early part of the game.
Neither of those two characters would be expected to be very active in the first stage of the auction, so that should all go OK. I guess the only change I would make to the timetable is that if Selina hasn't arrived by the time that you're due to do the thing that we list at 9:10, at the top of p18 of the instructions, it would be better to delay that until after she has actually arrived and people have registered her presence -- and give her ten minutes or so to settle in and work out who everybody is. That's like two hours into the game, though, so hopefully they will have arrived well before that!
Apart from that, though, I don't think you need to make any other structural changes: these games are pretty flexible, so as long as they are able to be present for a good chunk of the proceedings, it should all work out straightforwardly.
I hope that helps -- do say (or email me email@example.com
if you need to mention one of the game's secrets!) if there's any more detail we can go into,