Hi Lazydays26. Welcome to the forum.

Apart from the nurses, we're not medically qualified. I am not a doctor or any type of medical worker.

In general, not finding a tumour is good news - but it does leave open the question of what is causing yoru dad's symptoms.  I have to admit that I don't understand what is meant by "rogue" cells. I'm not even sure that "rogue cell" has any specific medical meaning. 

Fluid around the lungs, or pleural effusion to give it its Sunday name, is often caused by cancer, which is why the doctors are investing so much time and effort to eliminate this possibility. But it is not exclusively caused by cancer, it can be caused by quite a lot of other things. It sometimes occurs spontaneously for no reason ("ideopathic").

I'm sure the doctors will get to the root of the problem, but that means your dad will have to endure a few more tests before the answer is found. 

It's not surprising that your dad is worried about this. It's only natural. But really he has to hang in there and try not to let this get the better of him. You can help by keeping him cheered up. Support him through this rough patch.