Default [Q] Rooting Spreadtrum SC6820

Model - Symphony W15
Android - 2.3.5
Kernal Version 每 2.6.35.7
Software version 每 ZE25PBA_044_V8_0_5
Hardware version 每 V2.0 每 SC6820 (1 Ghz.)
Screen 每 3.5§ HVGA.
I tried several methods such as DoomLord, SuperOneClick, UnlockRoot, Z4Root,Framaroot,SRS etc to root the phone but none worked.
Tried ADB to manually root the phone through commands but almost every command returns 每 ※Permission Denied§ or ※Read Only System§. However files su, SuperUsr.apk, BusyBox were easily pushed into /data/local/tmp. Further steps like making local.prop and onwards could not be executed and returned above stated errors. Also tried modifications by putting update.zip on SdCard root and runningAndroid Recovery System but this method also returned error ※SignatureVerification Failed§.
Scanned my device with &X-Ray* app which reported that device or firmware I am using had been patched and is not vulnerable to any of the exploit available at the moment.
I desperately need rooting to remove pre-installed bloatwares and applications to free internal memory which is very less. I am trying to root phone for the last several days but every method has failed. Can any one help??