I haven't tried, but I presume I can also edit the files on the microSD on another PC to change the settings back to the original if I can't do it on the Yun.
You could do that if the PC knows how to read Linux style file systems. When you did the disk expansion, you created two partitions on the SD card: the system files are in one partition with an ext4 format, and the data files (/mnt/sda1) are in another partition with a FAT format. The files you would need to edit are in the ext4 partition.
A standard Windows installation cannot read ext4 partitions (though there are likely software packages available that can.) A standard Linux installation can read ext4 partitions. I assume a Mac an read ext4 parititions (but I have no experience with that, all my Mac experience is out of date and pre-dates the Linux based MacOS X.)
I have the impression that the bridge is intended to allow the Arduino side to access Linux facilities. At the moment I can't see myself wanting to do that. My concept is to use the Linux side to control the Arduino side just as I do when my PC is connectd to an Uno. However, time will tell.
Yes, the Bridge is generally driven by the sketch. However, you could have the sketch start a Linux process (using Process.runasynchronously()) and then that Linux process could communicate back and forth over the serial port. This is similar to what you are proposing, but likely not as efficient, but it still allows you to use the other Bridge functions such as having the sketch make outgoing web requests, respond directly to incoming web requests, or directly read/write files on the SD card.
You say, in your reply "In case we need access console again" which leads me to think that the modification to /etc/inittab disables a console program - or perhaps it stops it being loaded at startup?. Is that correct?
That line in inittab starts a program that runs a command interpreter on that port. It lets you use something like the YunSerialTerminal sketch to access the Linux command line when you can't get to the board over the network. By disabling that line, you still get the startup messages during booting, and you can still get access to uboot and preinit, but you won't be able to use that serial port to get to the Linux command line once it's up and running.
If so, can it be started and stopped from the command line like any other program?
Perhaps. But if you can get to the command line (presumably over the network with SSH) you already have a CLI interface, and you don't really need to start it on that serial port to get you out of trouble - you already have everything you would need. The value of having the command line on the serial port is to get you out of trouble when you have no other way to get to a command line. You'd be stuck at that point, since you don't have a command line where you could enable the command line on the serial port.
That's probably the main reason sonnyyu suggested the system expansion on to the SD card. You edit the inittab on the SD card, so when the system boots normally, the command interpreter on that serial port is disabled. But if you get into trouble, you pull out the SD card, reboot, and the system boots up with the original inittab that is stored on the flash file system on the board. Presumably, you didn't also modify that copy of initab, so the command line will now be present on the serial port.