Viagra increases the effect of nitric oxide, which is bad stuff when it gets into cytochrome complex iv in the mitochondria (IIRC). I would personally suspect based on that, that its effects on brain health would be negative in spite of blood flow.
2. There's no such thing as increasing "the effect" of such a nonspecific chemical as nitrogen oxide. There's tons of effects, and it increases some of them. Also, can the active ingredient get into mitochondria?