This should probably be in
YMDC rather than tactics.
It isn't exactly clear. Both the Wolf Tooth Necklace and Kharn's Gorechild both specify that they hit on a specific value. Invisibility also specifies a specific to hit value.
As there is no stated rule to work out conflicting set value modifiers it sort of comes down to how you want to work it out with your opponent.
I personally believe as both
WTN/Gorechild and Invisibility are advanced rules, neither trumps the other (Codex does not automatically trump rulebook any longer) so the only reasonable way to sort this, in my opinion is to either roll of to see which applies last or allow the player whose turn it currently is to decide.
This thread here in YMDC should cover the main points better
EDIT:
I'm wrong about Codex not trumping the
BRB. Last line of "Basic versus Advanced" details how this works
On rare occasions, a conflict will arise between a rule in this rulebook, and one printed in a codex. Where this occurs, the rule printed in the codex or Army List Entry always takes precedence