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Although not mentioned in the article, the trick can handle this. The rule should be "remove a digit that isn't 0", and if the sum of remaining digits is divisible by 9 then the removed digit must be 9 as well. That will cover everything except the case when the original number and the scrambled number are the same.

I did the same thing, starting from 100. I guess maybe "no digits" is a sufficient response to end up with 90?

yeah for first 10000 digits there is about a 2%-5% chance (around 20%-5% for first 2k, then falls below 2% for remaining 2/3s) of selecting one that doesn't work. obviously picking something like 1111, 2222, ... won't work either since you can't rearrange it to anything but 0s.

Your remaining digits are NaN

lol