I never said Natsu could negate Sema easiy, and I know its not heat-based. What I've theorized (like
@Brandish has) is that Natsu could possibly stop Sema in its tracks with a Flame Dragon King's Roar or Flame Dragon King's Demolition Fist, destroying it with that spell. If Natsu could destroy a huge war god, a meteor which is smaller than the war god should be able to be destroyed as well. And, while it mightn't be easy for Natsu to do it, I still believe he could do it.
As for Jellal holding Sema back to the end of the fight, whats to say he'd still be in the fight at all if it was long and drawn out? If Natsu lands even just a few hits on Jacob in FDKM or DF, its over. Jellal has next to no durability feats, so I don't see why he'd be much better in that regard now.
Look at where it was cast again. It was on the other side of those mountains, and you can see from the panel
@Jko posted, they were well away from the mountains. As for translation sites, Crunchyroll hosts the official translations, so if you're trying to discredit the official translations and say that a website's translations that can't even get character names right is better, then I'm really questioning that right now.
Yeah, it totally moves at "God Speed" when we've seen Laxus's lightning attacks move even faster than that for example. Does that mean laxus's attacks are greater than this "god speed?" The same speed or greater than we've seen people like Natsu have with some of their attacks? I'm not buying the "god speed" thing. Sema moves at a decent pace, but its no "meteor" like Jellal uses. And heck, Natsu could use a FDK's Demolition Fist on the Sema and destroy it. As for Sema's size, there is no evidence of there being more sizes so I think you're just grasping at straws here to try and get Jellal over.