Bluetooth problems and occasional freezing/extremely slow response with my iPhone 6s

Hi all,
I apologise if this topic has already been covered in the forum threads. I have a 6S, I use voiceover, and I am running IOS 9.2.1. I'll list the issues I am having below, and would be keen to hear from anyone with any constructive suggestions. Note also that the issues are still occurring after multiple resets, and exchanging my 6s for another at the apple store.
1. I have an apple wach paired with my phone. In addition, I was using a device for my hearing aids which bluetooths to the phone, and enables me to hear calls through my hearing aids. Occasionally I would get an extremely loud buzzing noise from the IPhone's speaker when switching the hearing aids device off. the only way for me to get this to stop was with a reset... the home + power buttons. this seemed to happen more frequently as time went on. It happened once more after exchanging the phone, and I have since stopped using the hearing aid Bluetooth device. I have never had an issue with using more than one tluetooth device, and was wondering if anyone had any thoughts on a way forward from here.
2. Again occasionally, but with increasing frequency as time went on, my phone would seem to hit a wall. The response times for gestures would lag to the point where switching off or a reset was the only way to use it to do anything. This has just started to happen on the phone that I got exchanged as well.
3. Occasionally the Voiceover voice which I am using gets staticky, and requires me to restart it with a triple click of the home button. Mildly annoying, but it would be fine if only it was happening on its own.
4. I like to think that I take care of my devices, and I can't think of anything obvious that I'm doing to cause these issues. I was told at the apple store not to close active apps from the app switcher, and an analogy was drawn with control alt delete. I was and still am puzzled by this advice, as I don't know of any other way to stop a spoken navigation app like blindsquare from speaking once I want it to shut up, or from a banking app to logout of secure information. Any thoughts, tips, suggestions appreciated.
cheers, Henry