Can I just clarify something - it's last names that must be alphabetical, right? In your example, after Pauline Wan, Anna Xander would be ok? The first names would come into it only in the case of two sims with the same surname?
That's correct. First names come into play only with Sims with the same last name.
I'm in! I'll assume that town editing is not allowed except for the Twinbrooks library, and adding a skill bookshelf.
Right. Thanks for reminding me of that exception.
Do we have to finish other challenges first, or can we do multiple challenges at once?
Finish any challenges in whatever order you choose.
Also what if you get to a letter and can't find a name to follow? for example you get to Pauline Wan, but can't find a 'X' last name, do you then find anyone with a 'S' first name, or (as in your example) does it have to be in the same family. Or is does it matter if its first or last as long as it follows the letter before?
Last name takes priority. As I mention above first name only applies to Sims with the same last name. If you would get to Pauline Wan and there would be no other possible targets for you, then you would simply bide your time until the clock ran out.
What happens if you accidently get a romantic interest not in alphabetical order (say from a free will interaction or mis-click, or someother mistake)? Do you just keep track of it and not count it at the end for the scoring, or is there a worse penalty (perhaps keeping track of it and having it count as a negative at the end, or even being disqualified from the entire challenge)?
I would not disqualify over a simple mis-click. But, yeah, a penalty would be assessed. If this happens to anyone, you need to note that this occurred and make full disclosure of how many times it occurred when submitting your final results.