Why would there be a random portal from SS to Kazui's room?? I didn't see a Senkaimon opening. And where do you have proof of this limit to Yhwach's powers?? He literally states in Chapter 680 as he is leaving For Soul Society that if the gang prove their fortitude ad pursue him then he will reward them by finding the greatest point of happiness in their future and rewrite it to destroy them. That is literally stated in the Manga.
And at that point in Time Ichigo, is happily married to Orihime with a Son, Rukia, with her husband Renji and daughter arrive in the Kurosaki clinic after her promotion to Captain. Judging that the kids don't know each other they haven't met in years, so this would be the perfect time to destroy them all based on his threat. His power was to rewrite the future, thus would need to send powers to the future. So yeah, your "I don't explain how I see it" doesn't work there bub.