The answer to both of those is "It's irrelevant.". Provided he isnt killed by the Courier (an unforseen chaotic in his projections), House has already displayed enough vision and foresight that neither of those two constraints will matter. Let me explain: first on the subject of immortally, he has already shown through his quest line dialogue that he can reasonably expect to survive at least another 100 years. In his own address to the Courier, his plans stretch that far. Once humanity has been salvaged and rebuilt that much, his place in it will begin to take a back burner to new innovation. In addition, although not strictly canon, there is a huge line of thinking that the reason Yes Man goes off line at the end of the Free Vegas line, is to incorporate "More assertive code" left over from Mr House into his program. It's completely reasonable to think House has a backup of his neural pathways, similar to his two Robo girlfriends. As such, he still wins just not with his own flesh and blood at the wheel. And on the subject to limited scope of power, it doesn't matter. He himself says "I couldn't save the world, nor did I want to.". He's much like Don Corleone: absolute power within the confines of his world. Vegas is just the legitimate front for his larger operation of rebuilding a world from scratch. NCR will fester and rot, similar to the brotherhood of steel. Caesar has major concerns of succession and long term viability. Perhaps the Enclave would have a good shot at rebuilding humanity, but at a terrible cost. Hell: The Master had a better long term plan in place that NCR and Caesar. Any way you spin it, House is the best answer, with NCR being the worst (long term).