Honestly, you should ONLY care about your 11th girl. That's the one with the highest multiplier. So just place 10 random girls (their multipliers don't matter) and then pick the girl with the highest multiplier as your 11th seat.
As for the event, I just finished the last girl today. It took me like 200 diamonds which I had saved up from tourneys anyways. Unfortunately, I think there won't be any way to get those event girls after the event itself expires.
side note: I didn't take it completely serious, misses out on a few chances to get more canes so I ended up having to spend diamonds. I'm pretty certain it would've been possible without those diamonds though.