At the risk of pissing off both the left and right on this board:
a) I am not sure why this consideration should even be made. Most very wealthy individuals do structure donations in the most tax efficient way possible, and I don't think we should ostracize those individuals for doing so. Most very wealthy people give for several reasons, the first is usually somewhat self serving and that the building of a legacy. The second is a real feeling of community and feeling that they want to give something back to those that need help. My guess is Trump's reasons are both (although I would guess the legacy is slightly more weighted).
b) Likely he has not received tax deductions for all of his deductions, and depending on how things have been structured, he may have received only a small portion of deduction relative to what was given. My guess is he has set up a charitable trust of some sort and the vast majority of his donations have been made out of that, but just guessing.
Long and short, like I said before, I feel as though he is our best (worst) option.