You can expect anywhere between 10-25% return with a hair straightener.
You may be able to add more coconut oil and have a similar potency in terms of THC. What you probably wont get is the other polar plant compounds the ethanol pulls out, which may have medicinal properties.
After, and yes basically can do what you said above.
On a hair straightener? A long time. I have a small press and can only do 5 grams at a time. It takes roughly 5 minutes between presses if I work quickly. So roughly a gram a minute.
If I was going to process 20 ounces for edibles (or capsules) I would probably use ethanol. Have you considered getting a still to reclaim the ethanol? I have an Source Turbo by Extract Craft and love it.
http://www.extractcraft.com/store/