Why can't you use 2 when you can use 1? Why can't you use possible,necessary, important after a person when you can use easy or difficult after that?
1.It is easy(difficult) to please him=> He is easy(difficult) to please.(0)
2.It is possible(necessary, important) to persuade him=>He is possible(necessary, important) to persuade.(X)
1a and 1b sound natural to me, though a 'difficult man' may not be the same as one who is 'difficult to persuade'.1a. He is difficult to persuade. 1b. a difficult man
2a. He is possible to persuade. 2b. a possible man
3a. He is necessary to persuade. 3b. a necessary man
4a. He is important to persuade. 4b. an important man
My feeling is that 2a, 2b. 3a and 4a may be heard in informal conversation, but I would not recommend them for use by learners. 3b is possible in restricted contexts.
4b is fine.
These are personal, rather than'rule-based', opinions. It will be interesting to see what others think.
How about the negatives of 2b? An impossible man is natural, though the meaning has changed. A possible man would, I think, sound natural enough in context, though I think more in contexts like possible candidate than possible to persuade.