Whatever way you are happy living your relationship(s) is cool, as long as you ARE happy with them. I couldn't do the alpha thing, but to each their own.
In answer to your questions...
1) I think that's an unhelpful way to have that conversation. How she brought up the subject seemed to be dishonest (though I have sympathy with her - it's a seriously challenging subject to bring up, especially to a partner who isn't going to like it) and seriously, there are better ways than over text. Though again, it's a scary thing. Text makes it less scary. If I'd found out that way I'd have been...shocked and a bit pissed.
2) I told my (ex) partner about it face to face, over several years, in several conversations. We ended up splitting because (mainly) my need to be poly was incompatible with his need to be mono. Both of my partners knew about it before we started relationships. Not making that mistake again.