[RPG] Can you repeatedly use Fortune Hex twice a day if you have the Accursed Hex feat

Probably.

There has been no clarification on the ambiguous "as if you had used Diplomacy" language. In general it's accepted that you can lay the Charm Person spell on someone, improve their attitude further with Diplomacy, and since those are two separate effects, when the spell fades they're left with however many levels of attitude improvement the Diplomacy did. As the Charm hex is of small benefit (1 level improvement) and short-lived (albeit a standard action and near undetectable), I see no reason not to allow it. Do note that the Charm hex works on animals and humanoids but Diplomacy only works on humanoids (whose language you can speak) so you'll need Handle Animal to get more out of animals.

Also if you're going this route please notice that before Helpful, there's additional checks required to get them to do things (with the Charm Person spell these are replaced with opposed CHA checks):

If a creature’s attitude toward you is at least indifferent, you can make requests of the creature. This is an additional Diplomacy check, using the creature’s current attitude to determine the base DC, with one of the following modifiers. Once a creature’s attitude has shifted to helpful, the creature gives in to most requests without a check, unless the request is against its nature or puts it in serious peril. Some requests automatically fail if the request goes against the creature’s values or its nature, subject to GM discretion.

I believe that a witch could use a Rod of Voracious Hexes along with Split Hex to target 3 creatures at one time.

The rod doesn't specify that the hex must target a single creature, just that the additional target and the normal target be within 30 feet. So, a witch targets creature A with a hex, uses Split Hex to also target creature B and the rod to target creature C; as long as B and C are both within 30 feet of A, all three should be affected. There is an argument to be made that all three may need to be within 30 feet of each other for the rod to work, if one assumes that Split Hex makes both A and B the "normal target", but I feel it's a weak one.

As a GM, I'd have no problem with this ruling, not least because the witch is paying 32k for the rod which only works 3x/day. It's extremely easy to blow through three uses in a single combat (one of the first things I found Googling around for this combination was to use the rod to hit three targets with Evil Eye then Slumber - which is a potent combination, but can only happen once per day).