So just because you can't hypnotise everyone into your bidding it's a problem.
First, I'm not hypnotizing anybody with Speech 100; what I'm doing is convincing them that my way is the right way (even if it isn't - and this alliance would definitely not be best, but it should still be an option). It has absolutely nothing at all to do with hypnosis.
Second, if a skill is maxed at 100, then you are without a doubt, without question, the BEST at doing that thing. The BEST as in: there're none better.
Third, if you are without a doubt, without question, the BEST at convincing people, then you should be able to convince House.
Fourth, I can convince everybody else into doing things my way. House should not be an exception.
Fifth, I wouldn't unite NCR and Legion because I gain nothing by doing so (in contrast to allying House and the BOS). But interestingly enough, my Speech prowess is enough to drive Lanius out without a fight. If I can convince a maniac to abort his mission to take over the Mojave, then I can (should be able to) convince House to do things my way.
I could keep going, but there isn't much point. This is my stance on it and nothing's going to change that (nor am I going to change anybody else's views of the matter). Mr. House svcks and I hate him and he dies in every one of my games and that's how I play and those are my opinions and... yeah.