Oracle – How to Prevent Updates to Database Before an Export Operation

Depends what you mean by 'from the command line', but you can generate the DDL via the dbms_metadata package (see here for example).

If you want the data as well then you'd then have to roll your own, unless you find a generic script somewhere on the web. Automating the script with dynamic SQL and the data disctionary is certainly possible - it's what SQL*Developer must do ultimtately - it will just need some work to get right, and to handle things like embedded quotes. No quick fix I'm afraid.

Is hard to say which method is faster, but I would say from my experience using source backup is faster than import.

It seems the issue you see with the file header when you use the source files comes from the file format of AIX which is big endian, while on Linux is small endian.

The files need to be converted. This can be done using RMAN:

RMAN> convert datafile 'bla' 
    TO PLATFORM="Linux 64-bit for AMD (or your platform)" 
    FROM PLATFORM="AIX-Based Systems (64-bit) - or your platform"

    (note: line breaks added for readability)