The best way to encourage the success of characters in this situation is to make sure they're using their abilities and items effectively. Some players put their ability cards away in a pocket and forget about them... but in CotP, these characters will need to use theirs, at the right times, and think and plan together about how to do so without being immediately thwarted. You can help this along, if they look stuck or baffled, by quietly asking along the lines 'have you tried doing X yet?' / 'have you any ideas about how that can be used to achieve Y?' / 'but what about Z? 'etc. For the host (or the author!), with a top-down view over the whole plot, it's easy to see how it should all fit together -- but sometimes it's difficult for players to get the right perspective on their parts of the plot elements. A little guidance usually helps considerably.
For casting, the best way (I find) is to think as if it was a film, what role would each character play. Clearly Hugh and Helen are the star romantic leads, so a lot of the focus is going to be on them. Shahfeez as a mysterious, sinister figure will also have a lot of 'camera time'. Lindsay, Ariadne and Robert are more subtle, behnd-the-scenes type characters. William, Alvy and Eva can be played either way as the player wishes -- they can be strong 'character' parts, or quieter schemers, according to preference. So yes, I think the way you have it at the moment should work just fine.