Because 128-bit means the data will be attacked in another way (ie getting access via the logon password, recovering the key etc, ie NOT brute-forcing the key itself. If that is the case 256-bit is pointless.
Trucrypt? No idea but it presumably won't be integrated on the fly as with FV2??
No, with FV2 it secures the data when shutdown, once restarted the security lies in the access password strength, although I have no experience on what will be required to mount a FV2 external drive on another machine, I'd hope the password and FV2 key as an internal drive can easily be removed from a machine and become an external.