Several questions regarding my Sony Xperia M4 Aqua

A

AC Question

1) The first issue is that, a couple of weeks ago, my torch had stopped working. When I open my camera app, I can choose several flash settings. Two of them are Fill flash and Torch. When I choose Torch, my flash should turn on, but it doesn't. However, when I choose Fill flash or Auto and proceed to take a picture, the flash turns on as it normally would to provide lighting for the photo. But it seems to me that the flash is different. The photos that I have taken with flash after my torch had stopped working look bad compared to the ones taken before my torch broke. I was wondering if there are maybe two separate bulbs, one for the torch and the other for taking pictures (maybe the one for pictures works at the same time with the torch one and that's why the pictures look worse now). This problem occurred before updating to Marshmallow.
2) The second issue is about my YouTube app. Basically, whenever I restart my phone (or power it off and on again) my YouTube app gets completely removed from my phone. That means that there's no home screen icon for it, it's not in the apps list and I can download it from the Play Store, which I do every time. Please note that I have recently updated to Marshmallow, but I've only just noticed the problem so I can't really tell if it started with the update or at a later time.
3) I don't know if I'm going crazy, but it seems as if my phone's memory just filled up without any apparent reason. 7,67 GB is used out of 8. I am running low on space, even though I uninstalled Twitch, Google Translator and a couple of other, smaller, apps (I had to uninstall them in order to install YouTube again, which bugs me considering that I had YouTube with all those other apps before). And now I can't even install YouTube again even though I had removed all those apps that I had concurrently with YouTube. And I've just noticed that my SD card's name has changed from SanDisk (it changed to SanDisk with the Android update) to what seems like a random combination of letters, numbers and dashes.
Big thanks to anyone who can help with any of these questions.