What do you think could have been a solid reason for Yhwach to not use Auswählen on Uryu in that moment he knew for sure that Uryu will work against him ?
I wouldn't assume that he has no need for Uryus power like Haschwalth stated - actually, at his peak of power it would have been more reasonable to regain "The Antithesis" from Uryu to eliminate a possible threat instead of taking the powers of the remaining Sternenritter to give himself a "boost" which hardly changed anything. Why he has granted Uryu with such a Schrift which could rival "The Allmighty" (atleast in theory) anyway ?
Or maybe it's just arrogance at this point, maybe he just thinks that any future outcome which contains the use of "The Antithesis" is not a threat to him ?
I think Yhwach didn't use Auswählen on Uryu because he couldn't. Uryu survived the first Auswählen despite him being Gemischt Quincy. Because of that Yhwach said that Uryu possesses sonething that even surpasses his own power. This would mean that even if Yhwach grants a power to Uryu he can't take it back. But i've got no real idea on the question why Yhwach granted "The Anitithesis" to Uryu in the first place. I just think that Yhwach must have seen something, that made him do it.
I always thought that when Sternritter drink Yhwach's blood, the power they receive is random and depends on some aspect of the person in question (Bambietta has an explosive temper, Bazz-B is a hot head, Quilge is authoritative and cruel like a warden, Mask is a glory hound).
If Yhwach really grants a power that is determined and developed based on the Sternritter's personality and such, or in other words a power he does not choose himself, I'd say he shot himself in the foot if Uryu really developed a Schrift power that can counter his own. Even more so if he cannot steal it from Uryu by Auswählen.
Although Yhwach is ridiculously overpowered and in the end may not even have any problem defending against The Antithesis.
Shouldn't the fact that he can't use it on Uryu or Ichigo be added as a weakness along with him temporarily losing his powers to that silver arrow and that he can't predict Mimihigi's actions (did I spell that right?)
Since everyone in Bleach really has multiple aspects of a single power, isn't Auswahlen stealing someone's future while Uryu's power rejects that future?
It would depend on how Uryu can use this power. Can he use it passively, so that he would reject Auswählen in any case if Yhwach used it on him ? Or does he need to know that Yhwach would use it on him at any moment or atleast a single moment of consideration ?
The fact that you ask these questions just shows how poorly written Yhwach was.
Your questions just reveal how aspects of the Yhwach character contradicted one another or just couldn't be reconciled with other areas in the story, setting etc..
I honestly feel that the character of Yhwach was just more a 'feeling' or 'style' Kubo wanted, or maybe Yhwach is Kubo trying to portray 'Fate'as Ichigo's greatest enemy as stated in the poem in the SOULs character book from way back, maybe at one point Kubo conceived the idea that Zangetsu would turn against Ichigo and thus we have Yhwach, the point is Kubo could only conceive and draw a very abstract character rather than a fully realised one.
Other than the obvious that he as one of the Schutzstaffel is not the target for the 2nd one,one thing to be noted regarding "Antithesis"(n Schrift in general) tho,all Yhwach did was give a "letter",that letter then become "word" aka the power(n even in some rare cases,the "word" grants no power at all),if I were to speculate I will say "Antithesis" is Ishida's own innate power that resist Auswahlen,all Yhwach did was give him a Schrift allowing him to use it actively,I mean come on,giving a power to your enemy that exist to specifically counter yourself is stupid,even by Aizen's level of cocky
Consuming Yhwach's blood is his way of controlling the sternritters, as it prevents them from developing powers on their own or using Letzt Stil which allows further independence from the quincy king and tightens the bonds of trust with other quincy, which are needed to return the lost power.
Just like the Lloyd twins where developing "The Yourself" from birth, Uryu was on his way of developing the anti-thesis because his grandfather groomed him to be independent of the quincy king, which caused Yhwach to hesitate to take his powers as a child; if he survived the auswahlen he would eventually be able to use his powers to reverse its effect (which he did anyway with the soul silver arrow Ryuken created). By giving him the schrift he made sure Uryu would come rely on the quincy king's power instead developing it on his own, which eventually would make his Auswahlen deadly on Uryu just like it was with Robert, Gerard and Haschwalth all of whom relied exclusively on their shrift to fight.