Multiplayer game. Interactions

[jilko]

Hi all,
Im thinking about developing a multiplayer OTA game. Im thinking about creating a virtual world where the user can go freely around it, moving with one place to another and interacting with other users (real users), chatting, doing some actions, etc.

I have several questions but I expose here these two:
1) What benefits does Snap Mobile give me under these circumstances? Should be better in that case building my own server?
2) Can somebody give me a hint of hwo can I deal with the user movements around this 3D Virtual World? I mean when the user press UP for three seconds, this order is sent to the server and it sends back to the user device the new position? Because if the device is in charged of calculating the new position, it won't take into account other user movements in case of collision. Same problem when I think of moving objects. Now I dont see affordable the problem of building such an engine for the movements.

Thanks in advance