No, Natsu didn't took Jacob down in one hit. See for yourself:
It may didn't do major damage, but it sure did damage to him. Natsu's FDKM attack
didn't oneshot Jacob, which you can't deny.
Jura is one of the best defensive mage from Ishgar. Tbh, he could be the strongest when it comes to defense. Rock Mountain is extremely powerful when it comes to defense, since it could tank Grand Chariot with ease.
Natsu isn't stronger than God Serena. Natsu isn't stronger than the Four Gods of Ishgar, he wouldn't come further than defeating two of them when he uses FDKM mode. God Serena > Natsu (I don't have any doubt about it).
Marin could undo his FDKM spell in a blink of an eye.
Anyway, Natsu will be defeated by Neinhart's corpses spam. Mard Geer, Hades, Zero, Jackal & Zancrow >>> Natsu.
Her speed and attack increases. You make it sound like she has zero defense. Once she is in Etherious form and her Enhancement is activated, she'll
oneshot others.
Neinhart's Historia of Corpses and Wahl's Weakness does affect space.
Kyôka is on a whole other level than Tempesta. In the end, Tempesta was fodderized by Laxus and Gray. Kyôka will use her Sensation Curse on Gray and he'll scream. Gray could solo Base Kyôka but won't defeat Kyôka without extreme high diff. I'd rank DeS Gray more or less equal with Etherious Kyôka atm.