I'm not an expert in oil burners. However, I'll throw in my 2 cents.
Your setup is not ideal. Better setup is having the oil line exit the tank, go down and then up to the pump and burner. That way air/ga/vapor would either go back up into the tank or go up into the pump where it would be a momentary "burp" and the pump would continue to get liquid. I will assume a change to this configuration is not possible.
I think you are on the right track with a de-aerator. The physics of the oil tank, supply line and pump locations in your set up leave the system vulnerable to air/gas/vapor build up in the supply line where it is above the level of oil in the tank and burner feed pump. The pump is near the burner, pulling oil from the tank. Hence, when enough air/gas/vapor builds up in the supply line, it will accumulate in the high part of the line, breaking the continuity of oil within the line and the pump will lose prime. At that point it will quit. Putting a de-aerator in the high part of the supply line should cure that problem. The Tiger Loop or similar device should do that.
It doesn't seem to me that changing the tank outlet from bottom to top accomplishes anything useful. In order to get oil out when the tank was getting empty there would need to be an internal tube that reached to near the bottom connected to the supply line. The inlet to the supply line would still be approximately where it is now, and the oil would still need to be pulled by the pump to flow all the way to the high point in the line before gravity would allow it to flow to the pump.
I also don't get the usefulness adding a return line, unless there isn't one (can't imagine it works at all without one) or if there is reason to believe that the existing one has some issues that make it effectively too small. Kinked or flattened are two such reasons. Only other reason that might warrant an added return line is if the burner and pump were changed since the original line was installed and the new pump needs the larger return capacity.
Good luck. Hope the foregoing is useful