All the suggestions one would like to make are already in the earlier posts of this thread. For the nadir shot, you should only have control points on the flat surface of the floor/ground. Without viewpoint correction selected, you will expect to get a "bad" optimization. Accept this result and then select VP for the nadir image only. A further optimization should now give a good result. If this doesn't work, upload jpeg copies of the images and your saved project file to your personal web space or a free file sharing site such as www.ge.tt , and post a link to them here.
Besides, you can mask the nadir shot to exclude area that is going to give bad control points.