Questions about rooting Xperia M Dual topic

Hi.... I have bought recently Sony xperia M dual C2005 and I want to install large games (asphalt8/mc4/etc..)
but there is only 1 gb in internal memory
I searched too much but no way to install them on SDcard without rooting
please if there is a way to install large games on xperia M without rooting help me

I have some Question:
1- HOW to root Xperia M without unlocking Bootloader.
1- can i use Sony updates (if announced) ?
2- can i unroot it in order to use warranty ?
3- can i backup my current android os and then if i have problem to restore that?
4- if i have a problem can i restore factory default?

Please I want a guide to root Xperia M without unlocking Bootloader.

Sorry for asking many things Im not experienced in neither root nor android
thanks