I don't have additional questions for you, but wanted to give you a bit of info based on a friend's experience. Her daughter got her helmet off yesterday, so it was an exciting day!
They took the helmet off for an hour each day. During that time, they bathed the baby and let her hair dry. They also washed the helmet and let it dry each day. It really helped to keep it from smelling bad.
It did not affect her sleep. They were really worried about it, but she adjusted very quickly. She was a good sleeper before and slept just as well with the helmet on.
I don't know if it can reflatten. Do as much as you can with the first helmet though. She said many times, the head will improve significantly with the helmet, but they will outgrow the helmet before their head is completely rounded out (or as well as can be), so many babies require a second, bigger helmet. However, she said insurance rarely covers the second helmet. So get as much as you can out of helmet #1.
I really wish we'd gotten a helmet for my son. At the time, I was stupid and vain and didn't want to admit he needed it. He's 5 now and, when his hair is wet, you can definitely tell that it's kind of flat.