Traveling Huangshan in February

There should not be too many visitors in February and you can have a leisurely visit inside, but some sights of the mountain are closed, such as Lotus Peak and West Sea Valley. If possible, it is suggested you change the plan and go there during March~April.
You are right. The ticket is valid for two days, but make sure you register after you get out on the first day and well keep the entry ticket.

It does say above that the ticket is valid just for one day. Where shall I register to prolong it for one more day?

You can register the ticket at the entrance of the scenic area. About the exact location, you can consult the staff when you arriving there. Or you can stay at a hotel inside the mountain area, then you don't need to go back to the entrance and enter in the next day. :)