I recently got a message on my phone saying my 1520 had updates available. I updated to 8.1 & honestly I am beyond disappointed. Not for nothing, but I don't know why everyone wants so badly to upgrade to 8.1. It sucks! The calender changed & I liked it the way it was before when I had 8.0. My camera changed with the 8.1 upgrade & I'm having a hard time understanding how to use it. I can take pictures, yes, but the settings, etc. are very hard to understand now. With 8.0 I could touch the screen anywhere & it would take a picture. Now, there's only a small spot you have to touch. With 8.0 I could turn off the flash whenever I wanted. I have no clue how to do it now. Honestly, I hate everything about 8.1. I enjoy my phone much less. And, what is Cortana & how come it won't allow me to delete it from my phone? I wish it would just go away! Is there a way I can go back to 8.0? I really loved my 1520 when I bought it. I've owned a 900 & I still own a 920 but 8.1 has ruined my 1520. What is so great about 8.1? I can't even scroll to the next day on my calender like I used to with 8.0. No! It stops at 11pm! My tasks are harder to access as well. Who designed this dog of an update? Give me back 8.0!!!
I didn't read every reply in detail, but I think I can help with the spefifc complaints in your OP.
Camera, you were using the MS camera, and now have the Nokia Camera set as the default. Change this in the camera settings.
Cortana can be turned off, it's the first option in Cortana's settings. But I would advise against it as it is only Bing with a different GUI and a few features added. The search field being at the bottom, on it's own would be a feature to have me leave it on, even on a smaller screen.
Calendar, swipe left/right to the adjacent day, when it stops at 11pm, or at any time.
Tasks, were hard to access anyway, (why are they not shown in the calendar?) I use Cortana instead, and adding tasks or calendar events is easier with Cortana than going into the Task/Calendar app to do so.