Does this fit under airplane seats?

Yes it will depending on how bulky you pack it. If the backpack closes easily then you should be fine. The backpack is very comfortable with an excellent padded backing and the side zip for laptops is easy to open and close while going through airport security.

Is there no waist cinch to transfer the weight onto your hips? I only see a sternum strap.

The bag does not have a waist cinch strap. The shoulder straps are fairly wide and well padded, which makes the bag comfortable to carry when fully loaded. It is comfortable to carry when it is moderately loaded for longer periods. The back of the bag is extremely well padded and ventilates very well, making it comfortable for long durations of wear in warm climates. The overall construction material is extremely heavy-duty, and with the zippers that allow the bag to be expanded or compressed, paired with the outer cinch webbing coupled with the g-hooks, helps ensure that the load less likely to shift while carrying. This bag compared to others of less quality material that may have a hip belt, is more comfortable to carry similar loads, but if your concern is the bag moving side-to-side during strenuous activity, then it may be worth looking at other options.