Re: Heroes getting weaker.
WanderingWizard: You can read
this post for (theoretical?) information about that.
Personally, I'm positive all heroes came from Scythe. I mean, besides the obvious connection with him and the first Archon/king/hero, he's just a badass. Not that that's much of a reason, but eh.
On topic: I have no idea what actually happens, but I don't think the blood of the hero and the blood of a regular person would just 'mix' per se, but that the heroic blood would just sort of take over the regular blood. If that makes any sense at all; basically the heroic blood is so strong it doesn't matter if you try to mix it with other blood.
Also, if you read the second half of
this post, it states that people could simply not be aware of their heroic blood, because they don't have the sort of need to awaken it as you have in past games due to something tragic. That, and it's possible the blood just manifests itself in different ways. So, it might be possible you're marrying a hero, just someone who is not the same 'type' of hero as you.
EDIT: Another theory might be that the hero blood is present in everyone, yet in some people, like you, it manifests, while other people are simply carriers of the blood.