Working on a high current USB charger
1 and 3 are the same. You likely don't have a load when you are measuring it at first. Most regulators have a minimum load for proper regulation. Once you place a load, it starts regulating in the stated range. The LMZ23605 requires a minimum load of 1 mA. Throw an LED + Resistor on there and that should be enough.


  ΔVO/ΔVIN Line regulation
  VIN = 12V to 36V, IO = 0.001A
  ±0.02% mVPP


And depending on the the wire length, size, and resistance, that would explain a drop as well. Undersized or crappy USB cables will drop too much voltage at a higher current load. This explains the 0.1V drop per extra device. A 0.2 Ω cable resistance (round trip), at 500mA, would result in a 0.1V drop.

You already know 2. Yes, you need the signaling resistor pairs on the D+ and D- lines, or an iOS device will not charge. You need to use the Apple Standard for it to work with your iPhone.