Thank you for reaching out and using Adobe XD. It looks like you are trying to make comments in the XD prototype link.
1.For the 1'st query, I see when you make a first comment to the link it has the submit button. However the replies of that comment doesn't have it. Although, i can reply by writting the comment and hit Enter after that.
2. For the 2nd Question, I am unable to reproduce the issue at my end on my windows machine. Is it possible for you to share the video for better understanding along with the OS and XD version of your machine?
3.For the 3rd one, have you tried pressing SHIFT + Enter to move to the next link? Let us know if that works.