2. In terms of hours needed to gain fluency, Icelandic, Estonian, and Polish rank way lower than a bunch of excluded languages
3. Chinese isn't really a language. https://en.wikipedia.org/wiki/Chinese_language
Please check out this list:
While it isn't comprehensive, it does a MUCH better job explaining how this works.
are more difficult than hungarian
Your list is based might be as your source gives it. But that source is just a list made a by a single person that thought these were difficult. Their guess is as good as anyones, so a questionable source imo.
I know people like to bash wikipedia, but atleast there it gets edited by thousands of people, so it is like having your exam being checked over and over, the chances for mistakes are much lower, then when it is just the thoughts of a single person. (I am not claiming everything on wikipedia is correct btw...especially in controversial cases..)
Though it will remain quite subjective, you cant really measure it, you can just look at certain factors. I dont think this is a realisticly representative list.
isolated languages with hardly anything in common with any other languages for instance. Languages where the prononciation seems very far from the written version. Languages with a lot of exceptions to the rule, with more cases etc etc