Enough Gas Exchange?
 

Would a tank with dual overflows get enough gas exchange from the overflows if I were to cover the top of the aquarium?

Are you running a skimmer? If you have a skimmer, you can usually cover the tank without any issues provided the skimmer is getting a good amount of fresh air. Simply running the air injection line to the outside of the stand will greatly improve fresh air exchange.

I have a cover for my aquarium but I leave a little gap for gas exchange, feeding hole and to stick my fingers in there to lift the glass if I want to get into the tank.

I have a skimmer on my tank and havent noticed any problems.

I would imagine that if you pointed your powerheads to create some surface agitation. As well as using a skimmer in the sump would provide enough gas exchange.

Rippling the surface does nothing for gas exchange, there has to be a break in the water for proper gas exchange.

I only have an Aqua C Remora at the moment so I don't know if this gives me a lot of O2 into the water. The sump itself would stay open top.

The Remora should do a fine job. It uses a spray injection for the bubbles, and it creates quite a few. I shouldn't think you would have any issues. If you do, blow a small fan on the skimmer so it gets more fresh air.

Correct me if I'm wrong but I was under the impression that surface agitation created by powerheads helps break up the film on top of the water for filter's to pick up.
Breaking this film in turn creates a larger surface area for gas exchange to occur.

Yes but there has to be a break in the tension of water for the air to mix into the water. This is why airstones work so well. There are so many bubble breaking the surface of the water helping with gas exchange.

One question that hasnt been asked is what stocking levels do you have in the tank? How big is the tank?

After reading your first post I spent some time researching the matter.
From what I've found, the general consensus seemed to be that airstones are one of the poorest forms of gas exchange as the surface area of the tiny bubbles are so small.