Google Now is a replacement for S-Voice, so no more S-Voice when you are on JB... Samsung may brand it somehow, but I doubted....
what do you mean fix the email client?
In order to flash a custom rom, you have to have a custom recovery installed.... no bootloader unlocking, it's already unlocked.... there are different methods to flash a custom recovery.... you can flash it with odin, or follow the root no trip / install custom recovery method, etc... just depends on what you're comfortable with...
I was told that the email client included with the S3 was not the default android one and had reliability issues. Most notably with exchange servers.
Also I am glad to hear that about custom roms and google now. With the asus infinity you have to unlock the boot loader to install roms, and that voids your warranty.
One more off the wall question. In the leaked tmobile jellybean rom, is there a hardware button to trigger google now? A big thing I like to do with my current phone is to use the voice control to call people while walking. But to do this you must be able to trigger the VC without unlocking the device.