Hello ioannisfromaddison,
I created an account here just to reply to your message.
I too am visually impaired and use the three finger double-tap zoom gesture a throusand times a day. Since iOS 8.x I have had the bug where the full-screen zoom function randomly switches to the window zoom function. I have also had a bug where sometimes the zoom function doesnt know which way is up, and you have to slide three fingers right and left instead of up and down to increase/decrease the zoom level. I have just learned to live with those, and no update to iOS 8 has fixed them
To answer you question about iOS 9, I too have the EXACT SAME problems you are describing. I am experiencing;
Sometimes three finger double-tap zoom will work fine, zooming in and out.
Sometimes three finger double-tap zoom will zoom in, but a repeat three finger double-tap zoom will not zoom back out.
Sometimes three finger double-tap zoom will do nothing, even after three finger double-tapping 10 times in a row.
I very often three finger double-tap and hold the second tap to then slide up and down to adjust the zoom level for that particular screen. For instance, I three finger double-tap hold and slide up and down to view faces on Instagram posts, as each photo is different in scale, etc.
Sometimes three finger double-tap hold and slide will zoom in but not immediately adjust the scale as it should when sliding.
Sometimes three finger double-tap hold and slide will zoom in but will bring up the Spotlight Search page out of nowhere!
Sometimes three finger double-tap hold and slide will do nothing, even after three finger double-tap and sliding 10 times over and over again.
As I said, I three finger double-tap zoom in and out thousands of times a day, so if even one zoom attempt goes awry it is jarring. Within 10 minutes of having the iOS over-the-air update, I was exeriencing hundreds of failed zoom attempts, leaving the new iOS update infuriating and mostly unusable.
Restarting the iPad several times did not help.
Restoring the iPad Air with a downloaded iOS 9 .ipsw file and NOT restoring any settings and setting the iPad up as a new iPad, meaning as clean as you can get, did not help and the zoom function bugs were just as present.
Restoring the iPad Air a second time with the above process did not help.
Finally, restoring the iPad Air with a downoaded iOS 8.x.x -- whatever the latest version is -- got me back to where I was before and at least I can live with the occasioanl iOS 8 zoom function bug.
Thank you for posting your issues here, as I figured it was just me and my iPad acting up on iOS 8 and totally screwed on iOS 9 leaving me to consider purchasing a new iPad.
I don't know much about .ipsw restore files, but I know that Apple stops "signing" older versions at some point, making it impossible to restore to a previous version. That is why I quickly went back to iOS 8. I will stay here until iOS 9 is fixed, even though I have no idea how one would be able to tell.
If you can, consider restoring back to iOS 8 on your device. As you've noticed, iOS 9 is not worth it yet for those in our situation.