Reportedly, The Shamutanti Hills is closest to the original material and yet, as you say, there’s plenty that’s the work of Inkle’s.
In fact, I think the only system that’s similar enough to the gamebooks’, is spellcasting, and even there, it’s not completely faithful; but then again, the way the original worked was probably never going to fly with contemporary players.
Sorcery! has a time rewind mechanic that, AFAIK, shouldn’t be in other FF adaptations, and which is supposed to take the sting out of failure. Some, however, feel it makes them too failure happy; personally I have tried to live with the consequences of my choices, I’m guessing because, for the time being, I’d rather stick with one version of the story, whatever that turns out to be, rather than try to sample all the options at my disposal, and so I ponder my choices.
I gave Sorcery! another try because of an itch I have for something immersive, and playing with outcomes would risk to break the illusion; doesn’t hurt that, so far, I have been doing decently, besides dying once because of some completely avoidable blunder while casting a spell.
I’ve been trying to avoid fights, partly out of simple caution, and partly because I remember the combat system to be hard to read: you’re supposed to look at your opponent(s) and figure out what move they’re about to do, act accordingly; in practice, it’s often a crapshoot, and since choices matter, that leads to more rewinding than would be necessary otherwise. That part is not so great, if you ask me.
As far as I remember, TSH is relatively plain: things begin to get interesting in Khare, and the third and fourth installments are supposedly where the game really comes into its own; don’t own those yet, though!