The impression I get starts with what happened when Griffith merged with Casca's Demon Child.
To me, that's Griffith's future Achilles' heel, the thing that will expose him.
They haven't "merged" as far as we know. But I do agree with you. The child will probably be key in dealing with with Femto.
And that is because Femto is virtually untouchable. Not even Ganishka could do anything to him. Skull knight with Beherit sword couldn't. Guts definitively can't, and he tried 3 times. Something different has to have the way to reach him. And we don't really know what the boy is, except he is somehow linked to Femto, he isn't human and is highly associated with the non-physical word.
Check, check and check IMO.
2) There is an assault on Elfhelm, and the party is forced to flee. If this does happen, I don't think they will be attacked by the Neo Band of the Hawk. My money would actually be on Ubik. That panel of him at the end of the Falconia ark looks like a war party to me, and where else would they be marching besides Elfhelm? Having Ubik show up offers some interesting implications for whoever is destined to use the Beherit. He is the one who convinces people to use the Beherit, and may be able to convince someone in the party to use it.
I can't really believe that. An assault to the island would require a large amount of time and apostles, wich we don't know if Griffith can afford. Also, the island is highly protected by magic barriers, creatures (the mermaids are basically scouts if they wish), magic users, the Elf Queen and expert apostle-killer crazy painless dog, Guts. That's quite a lot

However, if Griffith himself would go there... that's another story.
For now I believe Casca will recover, and she will stay there some time, alongside the group. That is because:
1. because an attempt against Femto is currently impossible as I mentioned;
2. she needs time to recover physically and mentally, including practicing the sword;
3. it's safe;
4. it's Guts' current mission. He had to choose between Femto or her, and here we are;
5. there's really not much back in the normal world to go back to, there is only Femto, his city (wich none in the group know until now[they will probably know with magic or a vision I believe]), Rickert and a monster clusterfuck.