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.
2/3/4. Doesn't matter, you can't convince someone with pretty words when they have a very VERY strong belief that their way is the right way.
Cause if we should then I want to convince the Followers to pick up weapons and fight for Legion at the battle of Hoover Dam.
I want to convince King that he should run Vegas and fight both of the NCR and Legion.
I want to convince Marcus to kill every other super mutant in Jacobstown.
I want to convince Rex to bite his backlegs off.
I want to convince that worker in Sloan to have six with his molerat.
Ridiculous?
I think NOT!
If we're gonna be able to convince House, HOUSE! That he should spare the Brotherhood then I want these options as well.
House is not going to budge no matter what % you in Speech.
There isn't any way to convince him otherwise cause you won't have any argument for this convincing.
Was a long time since I played House so I dunno if there is a dialogue option for this but if there isn't one then I think there should have been one stating "is there no way I can convince you?" or "How about talking to one of their leaders?" or "Can't I just convince them to get out of the Mojave during the battle?"
But he should shoot down every one of these ideas with no exception.
5. Lanius is not some crazed psychopath who won't listen to reason, Idunno what the dialogue line were for it but I think there is a good reason for him to back down through the dialogue line, he's brutal, not stupid.
[edit]
Spoiler http://www.youtube.com/watch?v=EBAHzsqBmCA