That depends on the finish you are using. Every manufacturer has their own "recipe" and you need to consult them for cure times. I would suggest a week.

It sounds like this is factory finished flooring. You could have the board removed and a new one inserted.

I assume the house was dry inside, and at normal living conditions regarding environment. Also that the flooring was in the house for several days prior to commencing installation. I did an installation recently. I installed plywood and then the new floor. I instructed the owner about acclimatizing the floor, which is the entire reason I took it in a number of days before installation started. The house was empty, and every time I turned up the heat to 70F, I would come in the next day to find the furnace off, even though it got rather cool at night. After installing the back room, I notice a couple of days later that the floor crackled when I walked on it! Very strange! Then I noticed the finish in that back room very slow to dry. This slow drying followed me through all 3 coats, but not on the front rooms of the house. There had to be a reason! I went down the basement. It was freezing down there. That cold, damp air was coming up under the new flooring. The finish in the front of the house dried only because it was all windows and the sun was heating up the surface of the wood. (This was not desirable either). So, what I am saying is, your problem could be environmental, and if so, it likely will settle down.

It depends whether the creosote is only on the finish or has penetrated past that, into the wood. I would try some mineral spirits first. If that is not successful, give a light rub, in the direction of the grain with some fine sandpaper. If the black mark is only on the finish, it will come off. If it has penetrated the wood, and if scraping the mark out of the wood itself does not work, then you may have to change the boards. I am trying to approach this progressively, from least severe to more radical as a last resort.

It depends what the current condition of your floors may be, and what kind of finish is on them now. If nothing has been done to them since 1951, chances are these floors will likely need a complete resand and finish. However, if you do not wish to hire a company like mine to do this work, and if the floors are wax finished, there is a product from Dura Seal called renovator that is designed to strip, clean and prepare a waxed floor for a re waxing. Polyurethane or varnish is meant to be buffed (abraded with a fine abrasive) and recoated with a similar type of finish. There are also specific cleaners for polyurethane finishes that leave no oily residue, or residue of any kind on the surface. Short of having such a cleaner, a mild mixture of water and vinegar may be used. Careful attention must be used that this is only a damped cloth and not dripping wet. Water and wood do not mix well.