At the risk of potentially necromancing this thread, it's on my mind with the recent developments with Shicca in EZ.
The primary obstacle to NaLu at this point, is Natsu's own thick-headedness. Now it would certainly be out-of-character for him to have some big romantic revelation of about Lucy, so there would have to be an outside stimulus that would catalyze it. The most logical way for him to realize his feelings for her would be for someone to do something to Lucy, most likely at Ignia's hand. The fact that we've now had two instances of NaLu doppelgängers being together, and the fact that it was Lucy who calmed Natsu down, a'la Katara and Aang makes me cautiously optimistic that it's in the cards.
Mashima once said in an interview that he didn't commit to a ship because he knew fans shipped Natsu with both Lucy and Lisanna (though he also said he favored Natsu with Lucy), and didn't want to risk provoking them. However, at this point, that doesn't hold much water considering his own actions. For perspective, he KNOWS that NaLu day is a thing, and has generally not missed in the past couple years; there can be no doubt he's aware how popular that ship is, and that he himself is at least not opposed to it. That might be part of the reason he teases it to hell and back, and despite people calling him a "troll" (sometimes duly), he doesn't seem like a guy who savors tormenting his readers with stuff like this.
The problem is I've read that Mashima himself declared in an interview or somewhere that, in spite of realising all of the things you've mentioned, including his own original preference for NaLu, he'd resolved, by the end of the main story, that he'd be painting NaLu as "more than friends and less than lovers"(don't mind me if it was rehashed earlier in this thread).
I'm not yet entirely sure he's changing this decision, even if I've heard that Ueda himself prefers NaLu?! What we just got was another of those typical weird close physical contact moments they've been getting for ages now(at least the post-cauldron moment in HERO'S- or whatsitcalled- was more symbolically intimate-the kind I'd never see casually happening between straight platonic life-partners- than any silly ecchi scene as of late).
And I'm still bugged by the way their more serious moment in ch 22/23 is somewhat cast into contradiction with the fear and pain she must've felt in FT when literally having his Fire Curse in her veins, making me wonder if I can take the recent "soothing" event that seriously, with all the emotional weight it's supposed to carry.
We're getting more clues towards Natsu not being ignorant at all of attraction or rather how attractive the girls around him are, he seems to favour Lucy in each and all aspects(in spite of that confusing harem scene at Selene's temple), but all the scenes where he's supposedly acting out of presumed (romantic) jealousy are either defused in a silly way or off-page.
Yes, I agree, Ignia or Selene herself might trigger something more serious if Mashima decides to take the ship anywhere concrete, but Idk if he really means to(I know what he did with RM, Idk what he aims with EZ, since I've only watched the anime S1, but it's still hit or miss with the FT MCs). Back in FT DiMaria NOS-ed all of his changes and partial END awakening when she tried to kill Lucy, Lucy is now his de-buffer from a similar to END rampage mode, and he seems far too entitled to decide that other guys can't get physically close to her, but with off-paging some guys getting beaten up for apparently trying to hit on her, all these and some of their old married couple shenanigans feel a lot like the Mashima/Ueda duo are just giving nods to the plethora of fanfic NaLu tropes.