Edit: I was like you, but then I grew tired of Jinx and Hex mico-management. And then I realized Grave Singer deals the same damage without Hex mico-management on enemies that can be critted and which you can hit reliably. And what a relief it was...
First of all, no it doesn't. Jinx does its bonus damage on every single hit. Even with the highest crit range in the game AND auto-crit, you're still doing plenty of hits without critting. And you mentioned targets that are immune to crits, or sometimes negate the damage through fortification. That additional hit is akin to having
permanent x2 crit on top of your normal crit.
Example math:
100 base damage, x4 crit vs x5 crit, both 13-20 crit range:
300 extra dmg from crit x 0.4 = 120 bonus damage per hit on average
400 extra dmg from crit x 0.4 = 160 bonus damage per hit on average
Increase going from x4 to x5 = +40 damage per hit on average
Jinx hit = 100 bonus damage on every hit (reliable damage, not reduced by crit immunity/fortification)
Result: Jinx adds more than double the damage that the extra crit modifier on Grave Singer does. And this is with the crit belt, without it Jinx is even better.
Secondly, I'm using Grave Singer on an ally, because it suits him to dual-wield handaxes. You fail to consider that a minor increase in damage on MC (and I argue that it isn't) doesn't mean an overall increase in effectiveness if your ally's power drops as a result.
Same reason I used Vanquisher on a companion in Kingmaker.
If you're too lazy to use hexes that's fine, I completely understand that. I personally really like using them, so it's no problem for me. I play in TB exclusively so there is that. And even when you don't use hexes, falchion is still an extremely powerful weapon type and way, way more powerful
by default than many other weapons.
Yeah, vs targets immune/resistant to crits, Jinx clearly wins. Though conversely, some enemies will also have Mind Immunity, making landing Hexes somewhat painful/wasteful (unless Shaman, I guess).
Also using BIS weapons on allies that can benefit the most from them is certainly a very valid and proper approach.
One thing to factor is they do not have the same base damage, therefore in case of Strenght builds with Size increasing effects, Grave Singer greataxe will deal noticably more damage per hit (3d8 vs 6d6 -> 5 more average damage).
Now you're playing an Aeon. Bonus sources of damage, like Aeon's Bane will work in Jinx' favor. Come to think of it, many Mythic paths have something: Holy Sword, Sun Marked, Vampiric Blade. As well as classes, like inquisitor, magus, warpriest, Spirit Hunter, Hellknight. Therefore, single target Jinx wins, you're right. Particularly vs bosses.
But... how does Wrath combat of optimized builds usually look like? You stand fast, wait for the hexer's round, curse the enemy, then engage, attack and kill one enemy and then patiently wait till next round, next Hex target?
Well, I don't know about you, but I prefer to charge the enemy and take advantage of their flat-footed status and my superior Initiative. Also, I'll often off 2-3 enemies in the first charge, sometimes more (on Unfair). Next round another 2-3 targets. Meanwhile I'm unlikely to Hex more then 1 per round (impossible without occupying the rounds of multiple Hex users, actually). And again, it means heavy mico-management.
And for such multi-target playstyle, for me Grave Singer, with its higher base damage and stronger crits, is better.