Sorry, but this does not entirely duplicate MC-82832, as that one makes no mention of the gateways.
Yeah, it seems the only Dragon that gets a health bar is the first one, even though in the past I'm pretty sure the ones that come from commands had them
Will do
This used to be true, but it seems this is no longer the case. I created a world with the same seed 3 times because I thought the same thing, but each time I got the same main island with the glitched pillar. Just try it.
Of course, if the End wasn't affected by the seed, then you wouldn't have consistent locations of End Cities between worlds with the same seed. Would kinda defeat the purpose since the seed is supposed to generate the same dungeons in the same places.