Maybe all of the underlings he'd trust with something as powerful as a logia fruit already have fruits.
It's not the kind of thing you'd waste on a low-level minion.
Absalom is just lucky Sanji doesn't know the mechanics of how the fruits work, or else he'd be a dead man.
I doubt it. Anyone he faces at this point in the series is going to know about Haki, so it's not like it'll make him invulnerable. And it'd take him years to develop moves for it.
Besides, I can see Luffy wanting to defeat Akainu using Ace's fruit, as a form of justice for his death.
The other possibility is that they learn how to feed it to an inanimate object.
Thousand Sunny the Fire Ship!
Although that might only work with Zoans. Cause that's the only type of fruit we've seen used like that so far.