If it helps, this is what Glazey said when I asked about the multiplier a couple of pages back (bolding mine):
Beyond the three skills maxed for the Hero Status, do the teens' other skills have to be maxed in order for them to contribute to the multiplier? The phrasing "unique skill level" suggests not. For instance, if by the end of the challenge the teens had achieved levels 4, 8 and 7 in Acting, the middle teen would contribute 0.08 to the multiplier (fair enough), but also that, if they had levels 0, 1 and 1 in Archaeology, one of the latter two would contribute 0.01, which seems a little... odd?
The three skills maxed for Hero status have to be unique - that means no other teen can gain that skill at all - it can't even show up in the other teens' skill panels. The skills after Hero status do not have to be maxed to count - each level counts. Your scoring examples are correct - the teens can gain levels in the same skills, but you can only count a skill for one teen, so, yes, it makes sense to count it for whichever teen has the highest level.
Also, with regards to difficulty, as the person who's apparently at the top of the leaderboard right now (?!), take it from me - this is definitely a tough challenge! For those struggling, earlier someone suggested practicing the first day a few times, and I'd like to endorse that on the grounds it's pretty much what I've been doing. I've tried five times now and only the third attempt made it past the first day, though I reckon this fifth run might go the distance too. Somehow my initial strategy is still roughly in tact, although I flirted with another tack for my fourth attempt and, gotta say, it did not go well! I've altered traits, priorities, lot visits, et cetera, all based on these dry runs, and having taken copious notes in my spreadsheet I've now got something that resembles an action plan... for, like, the first four days. Dunno about the rest of the challenge yet - I'll probably just wing it! But at least I'm increasingly confident that I've got a solid base to build on...