I think that we need more differences between Grace and Simon. I know that most of the particularities that we usually linked to all the religious aspect that Simon seem to have grow with, but there must be things besides that by the simple logic that they grew in two different environments, there most be cultural or social rules that they just don't know they are following.
Some examples I could thought:
Eye contact: Grace could have grew thinking that keeping eye contact while you speak to someone is a way to show you are listening to them. Simon could have grew associating eye contact with a way of intimidation. This could lead to A) Simon keeping prolongued eye contact in a effort to intimidate Grace who believes they are in the same page or B) Simon thinking Grace wants to intimidate him so he avoids meeting his eyes as a show of respect making Grace assume things.
Personal space: Personal space can vary a lot depending where you come from, and even when I find funny the idea of Grace been all over Simon, I believe that Simon would be the more clingy one. My reasoning is that Simon grew up in a cult in which everyone looked at each other as brothers and sisters, and lived in a space station meaning that there would not be as much space as in Earth, that led me to believe that people in Eden just didn't have a lot of personal space. Simon would be jumping at the start, but after a while he would be all over Grace.
Eating: Grace would, as we see in the book and movie, feel comfortable eating in front of other people, maybe even thinking that eating with someone is better that eating alone, while Simon could think that eating is a private thing to do, because food was something very precious in Eden and you were suppose to eat it by yourself in a safe space so no one could steal it from you. Simon when he realizes that Grace would actively eat in front of him doesn't know how to take it, maybe he thinks that Grace trust in him not stealing his food, maybe Grace is making a power move, maybe he is just an idiot.
Speaking: The people in Eden could have seen speaking as something reserve for when you have really important to say, or you needed to be very clear, maybe they spoke more through their action, because speaking can get your thirsty and water is a resource they can't just throw away in Eden. Simon been in shock by seeing how much Grace is able to speak. Grace and Simon having misunderstandings because Simon speaks through actions that Grace just doesn't get it completely
I also think it would be funny to see this differences been notice byt the eridians. If Simon gets to be part of the Hail Mary crew, Rocky would be really confuse to see that the humans can't even agree in what humans do.
If this is the AU in which Rocky meets Grace in space, but Simon arrives to Erid and meets Adrian first the scientific community would just be panicking because the things that the data they have of humans keeps contradicting itself.
