I wouldn't say all she built, just cut her ties to the Hellfire Club. That makes sense, because Emma wanted to build a school to train mutants, Namor could have easily supported her in doing that and he would have encouraged her to keep her surface world company going so she could build an economic empire to undermine the surface dwellers. I think the only thing she was going to cut ties with was the Hellfire Club.
Did Tessa work with Mastermind to erase her memories of Namor on Sebastian's orders? Or was she doing it because Xavier told her too? Since Tessa was technically working both sides at the time, working for Sebastian and Xavier both.
I would love it if they explored this further, what would the impact to mutant rights have been if Emma had been married to Namor all this time, she would be safe from any interference at all, she would only have to answer to Namor and he would only ask her to keep him posted on what she was doing to mess with the surface governments. I bet he would give her free reign as long as she didn't hurt any innocent people. Emma would take all the blocks out of Namor's head that Xavier put there, help him deal with his "voices" that have now come up in Invaders.
I think instead of Xavier / Magneto, it would end up Namor and Emma driving mutant rights from that point on and Emma would have the school she always wanted probably located in an island country like New Zealand where she trains mutants.
I would love a What if? of this, What if Emma Frost had married Prince Namor in her early 20's, how would that have changed the world?
She would be able to build her own version of Cerebro with the help of the top Atlantean Scientists.
Namor might even be more stable and cunning because his mental issues would be cured.
Emma would push Namor to defend mutants world wide, they might even form some kind of understanding with Magneto, or if Magneto got in their way they would take him out and take his Brotherhood from him!
I don't think this makes her look as weak as some people thing, it makes Sebastian look desperate to keep Emma and Namor from forming a permanent alliance because there was a very good chance that Emma and Namor together would have taken over the Hellfire Club and merged it's resources with Emma's company!