Regarding to your two questions, to me, the first is right.
"Would you go to dinner with me on Friday night?"
The answer are: "Yes, i will" and "No, i can't".
"Would it be right if we left after the mass?"
I think the sentence is rather unnatural. Is it a conditional sentence?
So the answers can be: "Yes, it would" and "No, it couldn't".
What do you think about my answers?