2 IPhone 4 Hardware Issues Apple store questions.

So Ive got a few questions hope i can get some help with...
My Iphone 4 (GSM Model/ software version- 4.3.1)
I still have my warranty and my apple care warranty.
1. The Power/Lock button on the top of the phone has become completely stuck (pushed in). Does opening my Iphone void my warranty? (I doubt ill open it myself). If i go to a Apple store do they have the tools to open and fix my button there at the store?
2. Proximity Sensor- Im not a tech guy so.... for some reason maybe when upgrading my software my Proximity sensor simply is not working anymore during my phone calls the screen stays lit up so my cheek pushes buttons. Similar question if i take my Iphone to a apple store do they have the tools or whatever is needed, to be able to fix or replace the proximity sensor there at the store?
3. With both of these issues with going to a Apple store will my Iphone 4 either get FIXED mechanically or REPLACED with another Iphone 4?
Thanks for reading any help is appreciated! thanks!