There appear to be two murder plots running concurrently in the text. There is the obvious one. The Merlin being manipulated into trying to kill Harry once again. This is probably the WC traitor (Peabody) stirring the pot. Magic ink and all that ...
I think "stirring the pot" (in ways that advance the BC agenda(s)... or just cause disruption/opportunity the BC is poised to exploit) is sufficient explanation. Explicitly making it a "Kill Harry" agenda I think is a mistake -- I think the BC is planning to
use Harry (and thus, wants him alive).
So long as Margaret LeFay's starborn child is alive, whatever plan(s) they originally formulated (before she went rogue, and ran away from her Whamp handler) are alive, too! The Merlin, of course, would like to see Harry dead; pretty much for exactly the same reasons the BC wants him alive: a dangerously-unpredictable "wild card" who's prone to black magic.
I don't think we know those BC plans with any surety, but I suspect they revolve around the idea that a Starborn would likely be the best tool for controlling a summoned Outsider; without a Starborn, that tool is likely to turn in your hand and cut you!
Then there is the second plot. The one by the Denarian's, possibly/probably in concert with the Black Council. This one is trying to either get Harry to pick up the coin or to kill him if he uses LC, by having it blow up in his face.
Definitely a Denarian plot; but I don't think it's to get Harry killed (neither by LC or by any other means).
I suspect the BC and the Denarians are maneuvering around, trying to use one another; I doubt they often get very much
done in any of these schemes, because both sides are to untrusting to commit (and each, of course, is fully-correct in their respective lack of trust).
If Lash sabotaged Little Chicago / hid Harry's own mistake from him with the intent to blow it up in his face, why was she busting her ass to stop him from using it? Why would she want to kill the host she still had every expectation of turning?
...
Based on Lash' willingness to go to extremes to stop Harry using the table, I wonder if she'd even noticed the flaw...
Exactly!. I go beyond "wondering" if she knew of the flaw -- I fully-expect that she did. I expect she knew much more than Bob about such enchantments, and spotted the flaw long-since.
... They had to empty the basement so someone could get in and fix Little Chicago ...
My main theory is that Mab herself fixes LC, entering via Lea's garden, which is a "back door" into Harry's lab for Lea & Mab, and very few others. At the moment, Lea is being a Sidhecicle, so
Mab herself is acting under the promises made by Harry's Faerie Godmother (and fulfilling promises is incredibly important to Mab).
Bob, in turn, didn't tell Harry either because (a) Mab had just demonstrated she could easily walk in through every defense, and she terrified him into silence; or (b) Bob's entire "terrified of Winter" schtick is a front, and he's actually been a deep-cover Winter agent ... ever since his creation!
(my seconary theory is that Future!Harry fixed it, since Future!Harry (a) knew about the flaw, (b) knew about Lea's backdoor, (c) as the owner of the skull, could give Bob the relevant orders. )