1) After a week of customization, I found that the phone is linked to my google account which I had entered initially. I don't really like this. Can I make a new dummy account and change to it without "hard reset"?
I have tried another a/c setup earlier on Galaxy Spica, it will sync but did not allow me to delete main A/C with Galaxy Spica, Not sure about I9003. But factory reset should delete the data and the Google A/C settings, may be you can then try another google A/C, but not dummy A/C.
2) If I do hard reset, what do I actually lose?
You will lose only data partition, your settings will be restored to Factory settings and any user applications that you would have installed from Market.
3) I have rooted the phone and can take backup with TitaniumBackup. Can I copy this folder to PC, do hard reset, setup with the new dummy google account, copy folder back to to the internal SD card, use Titanium backup to restore apps+data. Will I get back all my customizations like home screen and simi folders?
Just back up applications with titanium backup, for restoring them later. But you will have to install Titanium backup again, should not be a problem. You can also copy the SD card contents and restore later. OR Use ADB and back up the installed APKs. Nothing will be earsed from external SD so put back up on external SD or PC. Yes if you restore apps+data, you will get back app settings. Not the desktop settings and the Widgets. If you have LauncherProplus, You can backup the desktop shortcuts and customizations also to restore later.
5) Or should I copy the full Internal SD card contents to PC and paste them back after reset?
No harm in it, to be on safer side, It is recommended that you back up internal SD.
6) Is it possible to setup a dummy google account and then change it's password so that the phone does not have access to this account. Then if I setup my main google account as the second account, can I access the market?
No you cant use dummy A/C, You need genuine one. May be create a new one, dont use it for communication with the friends and family. As I see you have problem with the Gmail.apk. If you dont want this, what you can do is rename the extension of this apk as Gmail.apk.bak in system/app folder and the Gmail application will not be seen. I think your problem will be solved. The market will work without any problem. Alternatively, you can also back up the application and delete it from the System/app folder, which ever method you feel is easy and safe use that. Use rootexplorer for renaming the apk with Write permissions on system partition.
7) It is better to install all apps through apks so that they are not linked to my account?
Dint get your question. you can restore app using Titanium backup.
8) Is there any way to image the "internal" memory like we do on a laptop?
No, I donot know any method as of now without having custom recovery for I9003.
9) In case something goes wrong, can I get stock firmware (India) for my phone. My phone details are as under :
Samsung Galaxy GT-I9003 (SLCD)
Baseband version I9003DDKA5
Kernel version 22.214.171.124 root@SE-S605 #1
Build number FROYO.DDKB2
I too have the same FW. But it is not available for download yet on any of the Websites. So you can only gete it done at Samsung service centre. So better be careful.
Any other query you can also search in XDA and Discuss. Google is your way.