John Tullamarine and ROB-x38
That which you are overlooking is the resolved portion of lift from the few degrees of bank you are using to provide the horizontal force necessary to oppose the yawing moment being produced by the offset thrust.
Moment 1 - Thrust X arm to lateral centre of pressure, opposed by Moment 2 - Resolved lift X arm to lateral centre of pressure cancel out to give you straight flight but NOT along the aircraft axis. You can use parallelogram of forces here. Hence a small angle of sideslip which produced an opposite side force to cancel out the resolved lift sideforce. Total Drag balances the Total Thrust. Then a little extra lift please which with its extra induced drag added to the added drag for sideslip probably makes the two methods of slightly banked and wings level a push. This is one for the honours students doing their aero degrees to get their heads around.
Incidently weight and cg doesn't get a show in this area as it is all mostly in the horizontal and you cannot resolve the weight through 90 degrees. The implication of this is that Vmca is not affected by cg position contrary to what you see in many training publications.
Then you have a "form" of balanced flight and the navigator should estimate the sideslip angle to provide for more accurate navigation. Don't some handbooks provide an estimate.? I say "form" to indicate my reluctance to call straight flight while sideslipping balanced but indeed all the forces are in balance.
I guess you might determine the angle of sideslip without a vane on a boom by aligning a DG with an Inertial/GPS accurate heading reference and go asymmetric. You may see the difference between DG and Inertial/GPS when you stabilise.
Tell me where I might have lost my marbles. They are rattling around in my head!