It is possible some of them have been changed to airline to accommodate priority rows. Certainly Northern's 158s, while mostly in the original layout, have created priority seats by changing some table seats to airline.
If you book table seats the tables are mostly aligned (though not all of them). Might be worth searching for interior pictures of 156s online as a guide?
I'd ignore the orientations shown on Trainsplit, they aren't always correct (e.g. for TPE they are always wrong). The A, F or B should be correct though.
I'm fairly sure the partition (formerly separating smoking and non) in the middle of the coach on the pic I linked to is immovable, so the two table bays either side of that should be aligned if you're OK with a table. Or just take your chances on the day, I can't see a Thursday early morning being that busy.
From looking at Realtime Trains the front 2 appear to go to Oban, though I don't know what the coach lettering is!