That Ladder saw the need to qualify
basic divination is prevented by Hybrid Siphon supports my argument, I think. Implicitly it concedes that
advanced divination would still work on Lilly. I also think using Madam Silva/Diviners as the measuring stick is really selling Concordance short. Here's what Martin had to say on the subject during his info dump:
Concordance Ethea redefine reality, or fate, or something. Like I said, I'm not sure anyone knows. I've known a few good men who practiced it in my time, even they were not able to figure out much before the Tithe collected them. Diviners use it, but at a level so mild it just isn't the same.
Diviners seem to be, at least by Martin's reckoning, pretty low on the scale of Concordance ability - and on top of that we likely have to consider that anyone allowed to stick around in Harumph, like Silva, was not Tithed and so was likely deemed not all that special in potential. Some people - like Lilly! - may fall through the cracks, but in general by virtue of where we are we shouldn't expect to run into too many genuine powerhouses. Finally, we know that Senus is a Concordance user and whatever he did with that rose was dangerous enough that we burned it
and our own memory with Evie warning us not to pry into what we found. I mean, it could have been a Flow effect or something that doesn't require detection like an automatic defense system triggered on examination but still.
Honestly, given Ladder's assessment of NULLERROR, I'm not certain true invisibility from Concordance is attainable. Another concept I've been tossing around relating to it is "Connection/Relation" in that a lot of what we've seen about its workings seem to involve connections to people, events, consequence, etc. [Cleaver] may make a specific untangling of Lilly's future something to which an immutable answer is impossible but I'd expect someone of sufficient ability could likely approximate it as a set of probability distributions based on her past actions to predict her likely responses. In essence, I think it's less that Lilly represents a blind spot in Concordance so much as a knot or something. It'll ping as unusual to Concordance users, and perfectly untangling it may prove impossible because [Cleaver] prevents her from being locked into a single, unchangeable fate, but you can
see it.
