To have air in the return, when there is no air in the supply line, can only mean, (imo), air is getting in after the supply line joins the pump inlet.
Fuel after the actual pump, even within the pump body, is under pressure, and, (very), unlikely to vapourise, and form bubbles. But if it did, the bubbles are fuel vapour not air. Vapourised fuel would not get to the return line.
If their was a leak on the pressure side of the system, fuel would be leaking out. If that leak was within the injectors, then the leak would be into the combustion chamber, mixture too rich, smoke, etc.
If there was a leak anywhere else on the pressure side, air still cannot be drawn in while it's under pressure, (anything more than atmospheric). If pressure drops to zero, air can get in, but fuel must come out to make room for it, so it'd be visible.
We can see air in the return line, so there is a leak, which is allowing air to be drawn in. The leak is likely to be, (almost certainly imo), in the suction side of the pump.
Mac.