For the first point, while I do agree that Endsinger does come out a bit out of left field, I do believe that the 'true final boss' being something brand new was kinda predetermined as early as Shadowbringers main MSQ. Fighting and defeating Zodiark was inevitable, however then came reveal that Final Days are a thing, and Zodiark is what's holding it at bay. And with Endsinger being less necessarily a 'person' big bad and more 'anthropomorphized cataclysm' big bad, that's the only way I feel they could have dealt with Final Days without a rugpull on level of "Someone specific is right here on the star and was the cause of Final Days all along and Ascians never found it/dealt with it", which I imagine would have way more plot holes. So they kinda were in a corner there from the moment existence of Zodiark and Hydaelyn was made into a grey morality area, and Final Days were written as 'the true threat'. They could have potentially done longer buildup over patches/more expansions, sure, but I doubt that would have resolved the fact that Final Days (personified) would be the final boss in the end.
For the second point... Yeah, but also I don't feel like that's applicable to Endwalker nearly as much as it is to Stormblood/Shadowbringers, which both have had way more death baits on multiple characters. Endwalker's only significant ones were in UT, but, again, I don't know if it was anyone's experience, but I kinda knew it obviously wasn't death, and not even for meta "they wouldn't kill everyone" reasons and more for "there is a way to bring them back that was handed to me 15 minutes ago and I'm explicilty told they're still there around me, just incorporealized and dispersed" reason. So I never saw it as death and more... "putting a life on the line". Imo, it amps the stakes in even more ways because then it's up to you succeeding on whether they live or die, but that's more subjective view of the narrative. Ultimately, I do agree that several characters had great moments to die in the past (Y'shtola in SB, Y'shtola in ShB, Thancred in ShB, G'raha in ShB....), but didn't, but at the same time, for all of them as of current moment, I feel like they have done enough with characters for me to not feel like they kept them alive for no reason whatsoever, even if it took Y'shtola all the way until EW patch MSQ to actually gain personality and motivations.