I think 10 days is a perfectly fine length for the contest and don’t think it should be increased to 14 or 15.
There will never be any length of contest which allows everyone to participate for exactly the same amount of time. Time zones aren’t so far apart that some people are more advantaged than others in terms of how many weekends they have during that period! The only differences may be a few hours on either end, which a 14 or 15 day contest isn’t going to change at all, as days near the start or end of a contest could be considered more important.
The recent contests which have been delayed by a few days have really felt a bit too long. (And that’s despite me actually solving the hardest problem in that delay period.) Those people who need more time can happily continue solving them in the practice room. I could point you to about a million comments by people asking why their code is wrong as knowing so would help them learn - being able to access editorials and correct solutions after the contest has ended provides a means to do this, so stretching out the contest isn’t really advantageous to them.
As for the challenge problems - personally, I’ve never enjoyed them as much as problems where there is a definitive yes/no answer - that, combined with the fact that I know I have absolutely no hope of reaching the top 2, usually means I don’t even attempt them.
I don’t believe attaching a prize to the top challenge score is going to accomplish much though, as we all know who will win that Since the top 2 already have prizes, perhaps some prizes could be randomly allocated - eg, chosen at random from the top 10 or 20 challenge scores, not including the top 2 globally. That would definitely be an incentive for people to work on the challenge problem. (As would changing the prizes back from top 2 only to be spread out amongst more people, like they used to in the beginning.)