Hehe...sorry if I made things more confusing, jesslc. No, you do not have to add a lot challenge if you do not want to. You can still move to another lot as long as the one you move to has higher value than the previous one. But, if you add a lot challenge or if there is a preexisting lot challenge, that's when the moving restriction applies. You would need to deal with the challenge first.
I am now even more confused The current spreadsheet by Lena only counts unique lot challenges. Does it mean that the highest possible multiplier of challenges is 9 X .02 = .18? In other words, if we start on a lot that has, let's say, 'Cursed' as a pre-existing challenge, we have to complete it, and then there is no sense in adding this challenge to any other lot? (unless it's pre-existing somewhere again and we just have to deal with it)
The current spreadsheet needs to be updated. I have informed Lena of that, but she works quite a bit, so hopefully, she can get to it before Friday when this goes live. Quake Zone and Volcanic Activity need to be removed from Lot challenges. They are too random. You could choose them and have to wait a week for the event to trigger and I might only have to wait a day, so that's not fair.
But, to your question -- yes, only unique lot challenges will be scored. I have not computed the exact math, but once Lena removes those two challenges, you'll know what the max multiplier should be. And if you move to a lot with a lot challenge you have previously done, yes, you have to still deal with the challenge in order to move, but you will not be scored on it.