Nobody summoned the fetches, as I understand it; when Harry realises the phages are fetches, he realises they have to have been sent rather than summoned.
His spell appears to find Molly because she's the closest fit to being a mortal cause of them being there, because they originally targeted her black magic, rather than because of her summoning them in any way; his spell is looking for a summoner rather than a sender and it finds the somewhat awkward best fit. (Molly did nothing to choose them being there, but they are still focused on her as a result of her actions. This seems to me to follow the same pattern as what happens to Molly at the end of CD, in terms of Faerie power's interaction with choice and consequence.)
My issue is still with why the fetches went after Molly with his casting, but not before. If she were the beacon, they would have been coming to her to begin with.
Now, if she were an anchor, I could understand it. But there are too many coincidences with her not being there during the attack.
- Molly admits that Sandra told her about fear being a motivator to breaking addiction.
- Molly asks Sandra if Rosie took her pills; Sandra ensures her she has; what pills?
- Molly admits that she sent Nelson and Rosie nightmares to cause fear in them.
- Pell is attacked in the bathroom, which Nelson is also in.
- Molly is not with him at the time.
- A short time later, Mouse detects black magic on Molly.
Sp: Molly could have cast a fresh 'nightmare' spell on Nelson, which she didn't realize was summoning the fetch.
- Molly leaves Harry while he's investigating the scene of the crime.
- Another fetch materializes around the other friend she's sending nightmares too.
Sp: Molly once again left to cast the spell, which once again summoned a fetch to her friend.
- Harry sets up the reroute spell to cast back at the summoner.
- We've seen his spells fail before because they weren't correct (GP)
- He notes that its taking longer than he expected for the summoner to act again.
- The last attack doesn't commence until after Molly gets home.
Sp: She cast another spell once she was home, still not realizing she was causing the trouble.
- Harry's spell directs the fetches back at her.
Sp: Someone intercedes with the fetches, having them bring Molly back to AT rather than simply kill her.
The argument that someone was sending the fetches rather than summoning them is never confirmed. Harry speculates it, but that's before he knows about what's going on at AT. And he says that fetches are grown-up, powerful phages. They could most likely still be summoned.