So can I safely say, the FMC will fly a Great Circle track from point A to Abeam point B and another Great Circle Track from abeam point B to point C and the distance travelled is THE SAME as the Great Circle Track from point A directly to point C ?
Not only will the distance be the same, the track will be the same.
You are essentially asking (albeit in a spherical geometry): if I walk in a straight line from A to C, stopping at a point B along the way, will the distance travelled be different from walking directly from A to C without stopping?
In other words - whether or not you program it in to the FMC, you will ALWAYS pass the point B that is both abeam to the fix and on the Great Circle Track from A to C, and you will always travel the same distance regardless. And unless there is a bug, the FMC will know this.
In fact, you always pass over infinitely many other points along your track. Each of them are, in turn, abeam infinitely many other points lying on a line through that point and perpendicular to the Great Circle Track.