UPDATE: I would like to close this (my own issue) until I can investigate further. I believe the slow spawn rate may be because I built the spawn platform very high up. The endermen not dieing is because most of them are able to slow their fall somehow in the 3x3 glass chute. I watched them doing this after putting an endermite and watching them fall. This part I don't understand but will look into it further.
------------
I built a 50 block high platform 150 blocks away from the main ender island over the void. the platform has a chute to the platform below (the kill area). The platform is the only spawnable location until the main island. The spawn platform is 31 x 31 blocks, with a 'rail' of slabs around the perimeter. Standard endermite-based endermen xp farm setup.

Issue 1:

Using myself in the role of the endermite while working out farm design details, I lured the endermen w/ eye contact. The endermen will die if I hit them before they fall in the chute, even just hitting them with a torch or slab.

However, if they fall just because they are trying to get at me (eye contact but no physical contact), they do not die. To kill them after they fall takes multiple hits with a diamond sword at sharpness III. So this is a re-opening of MCPE-25008 which was closed as un reproducable.

Issue 2:

Only 5 enderman will spawn on the platform. No new ones spawn until 1 of the five die.

If I kill a couple, new ones will respawn, but the count never goes over 5.

If I stare at all of them, and they fall through the chute (and survive, see issue 1), no new ones will spawn.

If I go down and kill 1 of the 5 with a sword, then come back up to the platform, 1 will spawn. If I kill 2 downstairs, 2 will spawn when I come back up, etc.