Ok I was assuming you had one owner. Since that isn't an option, update your code to use the following line:
post.SetValue("NodeOwner", user.UserID);
Using the above over using this
post.NodeOwner = user.UserID;
will allow you to bypass the "read only" option of the strongly typed property and set the value. If this doesn't work when you insert the page, do an update immediately after you insert it and it should work without issue.