can not remount system using adb remount

[maheshchavan]

In my previous thread, I have written that I need to start adb for replacing wrong build.prop I placed with original build.prop After lot off struggling I could connect to my mobile using adb.
But then I found that I can not push build.prop on /sytem because it is read only. I tried to use adb root but it doea not permit. I used adb shell but can not su. My mobile is already rooted. But now terminal emulator is not working and adb is not allowing me to mount system partition using adb remount also.
How can I mount system read-write ?
Please help.
Thanking you.

 

[maheshchavan]

Clarification of the problem :
I wanted to change my device model, so I copied build.prop from another mobile to my mobile. I am already rooted. But unfortunately I did not change the o.s. from gingerbread to froyo in build.prop ( I have android 2.2 FROYO) When I rebooted, there were many errors. It is now showing no sim card. Mobile network search gives error. Effectively, I can not use the mobile. Terminal emulator, X-plore etc. stopped working. Only Rom toolkit free version works and astro file manager works. Other programs are forcefully closed. Needles to say, I must replace new build.prop with new one.
Using adb I can only pull files but can not write as it is in read only mode.
adb root and adb remount works only if you are in recovery mode ? Y mobile does not have any custom recovery so when I boot in recovery mode it hangs.
Superuser has installed su in system\bin directory. But from adb shell I can use su also. It gives permission denied error. Hence I can not remount the system.

Please tell me, how can I mount system read-write. I can install root explore as google play and even internet is not working.

Please help.