Except we don't have a clear picture of what Maeve was really up to ...
No, we don't.
It looked like she just wanted to
Beat the Queen (defeat -- and
impress -- her Mommy Dearest). There could easily have been more to it than that; it even seems likely. But we don't know.
There's also the issue of Nemfection -- what was
Nemesis up to? Was Maeve even aware of it? Maybe all the disruption Maeve caused was the bulk of Nemesis' plan (never really expecting the "jailbreak" to succeed). Maybe the jailbreak
was the main plan, and the murderous Mantle-swap just a last-ditch scramble to salvage
something, and an "F-U" aimed personally at Mab.
We just don't know.
... As far as the Former goes, they weren't really a surprise...
The Fomor were well-known.
I think Ethniu -- and the Eye -- were surprises (at least to most there).
... but it is a bit surprising how easily they were defeated considering.
Remember: Harry hauled out
TWO major artifacts. These were items that (to Harry's perceptions) outclassed the Swords of the Cross (which apparently have actual
Angels in them); so "how easily they were defeated" is "not
at all easily."
There is every sign that -- if Harry hadn't had the Spear to pull out at the last minute -- Ethniu and the Fomor would have won.