It's possible. It's possible but very hard to be sure. The only way to know would be to ask him and of course he probably can't answer at this stage of his life. Sudden changes of pressure in the eustachian tube from yawning or swallowing can produce sudden sharp pains in the ear which by themselves aren't really harmful, but prolonged fluid in the middle ear may cause hearing loss over time. Check it out.
Doubt it. New acute infection- yes, but fluid itself, no. Frantic screaming after going to sleep is more likely a night terror. With these they don't respond when you first touch them and seem oblivious. Kids with nightmares usually have them in early mornings and they respond to you immediately if you touch them. Often seen in kids with erratic sleep patterns leading to a sleep deficit.
Yes. If you mean if there is pusy discharge from the ear, the answer is yes. If you mean fluids from bathing or swimming, unless there is ear tubes in the ear, it shouldn't cause any pain.