Minecraft – How do protective enchantments work on equippable non-armor items (elytra, mob/player heads)

I highly recommend Etho's excellent subterranean spawning pads with a water-based mobevator for moving mobs to the surface. An ammended tutorial for the mob-spawner and transporter can be found here.

The video doesn't include a mob-killing mechanism but there are multiple examples on his youtube channel. Personally I prefer transporting them a bit to the side of the mobevator, then leading them up a 2x2x4 tube of water with signs creating a gap one tile up from the ground. The mobs swim to the top and drown in the enclosed 2x2x2 cube of water, while their drops fall through the gap and into the bottom 2x2x1 water stream that you can channel wherever you want to.

[Edit: I forgot to address your requirements.]

Easy to build? It requires nothing but pickaxes and a bucket. Glass makes it easier to construct but isn't necessary.

Completely reliable? I have never seen a clog, ever, nor has it ever failed to kill a mob.

Good item-gathering? 100% recovered drops, no lava or cacti to destroy them and you can channel them to a single tile anywhere you want with the output water stream.

I have invented a solution to this problem using a mechanism to apply one tick of lava damage. It occurred to me that a tripwire could be placed immediately above lava in order to remove it (using a dispenser) as soon as a mob hits it, thus preventing the mob from continuing to swim in it. Doing this for the water as well as the lava means that no acceleration is needed to make sure the mob will fall down, so the whole mechanism is only a little bit of extra height above a standard fall-damage trap.

Build details, from top to bottom:

• The zombies are pushed by water into the drop pipe. I don't know how important the length of the upper section is.

• The water and lava mechanisms are identical:

  • Each dispenser contains an empty bucket.

  • All pistons are sticky.

  • The tripwire above the fluid is the input to the left (in first image) piston monostable (rising edge detector), which causes the dispenser to remove the fluid into the bucket. The monostable ensures that the tripwire input will not interfere with the second fluid-restoring pulse.

  • Behind the dispenser is a comparator followed by 2 blocks of redstone wire; this detects the dispenser having a lava bucket (which counts as 1 stack) instead of an empty bucket (which counts as 1⁄16 stack), and triggers the right piston monostable to remove the fluid. In the event that the state of the dispenser gets reversed, this circuit will simply not generate a pulse, thus correcting the problem.

• Below the water block is exactly 19 ½ blocks of empty space (counting the sign) down to the fall-damage landing platform.

Results: When the zombies hit the platform, a few still take 2 punches, but most of them take 1 punch to kill. I have successfully operated this mechanism in my survival world fed by two zombie spawners.

Glitches:

• The amount of damage done is not completely reliable.

• It is possible for the dispensers to get inverted (resulting in zombies falling on fire, without taking any lava damage, or from the wrong height). This latest revision with comparators can self-correct this problem, but it still may result in an occasional zombie with the wrong amount of health. (I have also seen a dispenser completely fail to respond to pulses, so I suspect there is a Minecraft bug involved.)

I know it can be hard to build mechanisms from screenshots, so if you would like any further explanation of other camera angles, feel free to ask me to add them.