Wow, informative to see a bit more of the background, and also get a true view of how deep Anthony's twisted determination and willpower runs. There have been hints for quite awhile, but now we see he is bone deep born and raised as an absolute, "ends justify the means" machiavelli figure, a true fanatic. Perhaps with a bit of poetry, he himself is a direct of dark Mirror to the Shadows House, whereas Kate straddles the line of light and dark.
This puts another set of pieces into place though, while deepening the challenge in another way in that there are now very much 3 active sides. Kate's ace to some extent is that she very much has more of a valid claim then he does, both in her birth and in her methods and history. Anthony now looks pretty irredeemable, killing his father like that vs just sitting him down and telling him he was taking over is a bright line to cross. And while they may have an alignment of interests in taking down the GGF and getting to the bottom of it all, there is no easy way (if any way) where he'd come on-side. Probably the best ending for him would be to die during the final act.
Still, this does mean there are some external resources potentially available to Kate too, and ones that were in a certain level of despair that she may be a figure they can rally around. Another part of the puzzle, though the system covering the entire rest of the island remains daunting.