My iPhone 6 has turned into a potato after iOS update

I can't click the phone application in order to make calls, listen to voicemails, and add contacts. It freezes my phone everytime I open it and then it crashes. My phone has slowed to exponentialy slow speeds, it freezes almost every 15 seconds when I'm using it, my battery drains from 100 percent to 10 percent within 30-45 minutes, it's ridiculous. I updated to iOS 11.2

Can I get information about a phone I used to sign into find my iphone?

I recently met a cool guy at a hotel pool, and we talked for a long time. So long that I didn't realize my phone was stolen by a chair nearby. I used his phone to track my phone using "find my iphone". In the confusion of my lost phone, I had to leave quickly and didn't get any contact information. Is there anyway I can use my account to find information about the phone I signed into find my iphone on?

Can the Apple Watch series 3 with cellular be used as a cellphone?

I'm looking at the idea of replacing my Motorola with an Apple Watch series 3, however i can't find a source that says wether or not you can actually put a SIM card in the device and use it as a phone (they just talk about piggybacking off of your iPhone plan) does anybody know if just using it autonomously of an iPhone is possible?

How do I undo the IOS 11.2.1 update?

My sister updated my software without my permission. And I know that when you do an update it slows down. Not only that but every time I plug my iPhone into my usb on my computer it doesn't recognize it. How the hell do I undo it without my computer?!