#1 Is the dongle necessary for audio from the xbox? With at least 1 model of the LS the dongle is only needed for chat, the mic, to work.
You could use the headset as basic headphones by connecting the 3.5mm cable to your controller, instead of using the dongle, but it's not optimal and will drain your controller batteries much quicker.
#2 Can you use it for gaming on your PC while using your phone's Bluetooth?
Yes you can use the headset for gaming and the phones Bluetooth. Just turn on Bluetooth and connect.
#3 Can you hear your own voice when using as a phone?
Not really, just a tiny bit, only problem is if anyone is in your game chat or party, they can hear your side of conversation.