I believe the book you are referring too is Dead Beat.
I think your theory works if any Mortal practitioner can in fact summon Outsiders. ... The closest example we have is the ritual in Blood Rites.
why infect the Athame?
she couldn't have done it with nothing unless she wanted to be in considerable debt/obligation (which only Harry is stupid enough to do).
7) It also seems to have spread to the White Court causing them to change their diets, to the Red Court causing them to to become hostile openly and causing inner turmoil within, to the Fomor - active and aggressive after so long. Unless of course Maeve lied and these were merely moves in a much greater plan...
It was Dead Beat.
Summoning Outsiders
While I wouldn't say that any mortal practitioner can summon an Outsider, Harry said that only mortal practitioners can summon an Outsider. The Blood Rites example illustrates several points. A mortal practitioner of at least Madge's skill and talent (likely even Trixie's because Harry suspects she had been performing the ritual) can draw power from an Outsider. He Who Walks Behind is an Outsider. Harry "banished" him. He's back after the ritual. His power was active Inside as soon as the first ritual was performed.
So I would conclude that just about any mortal, with sufficient knowledge, can summon an Outsider. (I can break down how I reached that conclusion in more detail, if anyone cares).
The Athame
It could be that Nemesis needs some kind of vector to infect. So Elaine, or whoever, performs some ritual and Nemesis occupies the Athame much like the loa that inhabited the Cabbage Patch Doll. Then the target must interact magically with the vector. I neither advocate nor object to this idea. It's just what I thought when I read "why infect the Athame."
Elaine's Considerable Debt
Elaine had considerable debt or lied about it.
A Much Greater Plan
I subscribe to the "Much Greater Plan" theory. There have been several hints (perhaps misdirections) that those Outside have some sort of hierarchical structure. I believe Harry states at some point that the "Outsiders" are "foot soldiers" of "Old Ones" banished from reality. The Walkers were described as knights or generals; I don't recall specifically. Nemesis was described as a "sapper."
There is some dark and evil conspiracy behind basically every case file. If it is the forces of "Outside," I don't think Nemesis is the top/only part of that dark and evil conspiracy.