In what ways can I replenish “salt”

Despite promises to the contrary, "1999 mode" is more like one difficulty notch above Hard.

The things that are the same:

• The plot/story/dialog is identical.
• Fixed spawns are identical, you'll find the same weapons, ammo, gear boxes (although the gear is randomized in every difficulty), infusions, etc.
• Enemies spawn in the same locations, in roughly the same quantity as before. The harder enemies are definitely the same, although the weaker ones are a bit too numerous to be compared easily.
• The weapon and Vigor upgrades are identical and cost the same amount of money. I haven't noticed a large difference in my money supply. Any suggestion of "combat specializations" that must be tuned is puzzling.

The things that are different:

• Checkpoints are similarly spaced, although I've been told there are extra ones before the Handymen (the really hard "boss" level enemies in the game).
• You still get a shield, but it takes fewer hits and recharges much, much slower.
• It takes more shots to down an enemy, but bullets are somewhat more scarce. This makes weapon upgrades slightly more important, but I honestly have had very few situations where I ran low on ammo, and with your "companion" handing you weapons, it doesn't seem like a big deal, honestly.
• In lower difficulty levels, you can get a "hint" in the form of an arrow pointing towards your objective by pressing a button. This "navigational aid" is disabled in 1999 Mode
• If you die and don't have enough money, you will not be revived. You'll have to restart from a checkpoint. (Although, given the high demands on your funds, I'd suggest restarting from checkpoint if you die regardless)
• You can't adjust the difficulty from the menu. This isn't that much of a problem unless you plan to go after some achievements after beating 1999 mode that would have been trivial on lower difficulties - you'll either have to endure 1999 mode for them, or start over from the beginning on another difficulty.

Overall, it's very similar to other FPS games on their hardest difficulty. I'm doing pretty much the same things here that I would in a Legendary run of a Halo game, or a Veteran run of a CoD game.

You can unlock it with the Konami code at the title screen, or by finishing the game on any other difficulty level. Many games have this feature of locking out the hardest difficulty until you've finished the game, and I would highly suggest you take this route. There are times when you will want to know the lay of the land in a situation where you can screw up and still survive. Knowing what weapons to upgrade is harder without knowing what weapons you'll encounter, for instance.

No, which makes me feel a little bit sad.

There is only few cosmetic changes (for example with like with Cornelius Slate) but you can't change ending as far as I know.

From Gamespot forum...

Look up for the Gamespot Lounge interview with Ken Levine, he says that 2k wanted multiple endings but he said "nope".

Also you can watch on Youtube interview (4 - 6 min) how Ken Levine say that they wanted to "make player experiance that he had to make choices" but also he tells that this choices not change plot.