Your first two steps seem redundant, I'd think redirecting the ritual would qualify as Interference.
Maybe. Lash managed to redirect the attack on Harry, but that could be because she was redirecting it to herself, because Harry let her, or both.
On the other hand, in
Proven Guilty Lasciel's shadow had no problem interfering with Harry's attempt to track down Molly, and only stopped because Harry said he'd go through with it anyway and kill himself. So you'd have to protect against that, at least.
You're right though that this could be one step rather than two, though--although the social/mental conflict could be to stop the shadow from messing with the
block until it's too late, if one wanted to include it.
and if the 2nd person has an invitation I highly doubt it would violate any Laws as it's the equivalent of a requested medical procedure. Not all Neuromancy is Illegal, it's just generally frowned upon by stuffy Council wizards as being close to a Law, just like ectomancy.
I think the person throwing the attack should take the lawbreaker power--or at least change an aspect--because of the risk they're taking. Even if the target is okay with it, it doesn't change the fact that you're willingly chucking an attack at someone's mind that will likely do permanent damage.
I think the main different is that I dont see why the infected would need to be the one to "redirect" the attack if those casting it are capable of hitting their target, the infected would just need to devote everything they have in an opposed contest against the shadow to lock them down.
I thought that it should be the infected because they're the ones that chose to have the shadow in the first place, so it would make sense thematically that they'd be the one who has to get rid of it. I also have trouble imagining that a person who isn't the infected could tell which parts of the mind were whose, so I'd definitely up the difficulty significantly if someone else did the targeting--and I'd probably force the formerly infected to take some level of consequence just on general principles.