I think your reply should really depend on the costs claim, and whether you''ve given her a written confession of adultery in the past.
If you have given her a confession of your adultery in the past then you''re pretty stuffed, you might as well answer "yes" and deal with any costs claim that comes.
If costs are claimed and you have not given her a confession, then there''s no reason to play ball. She will never be able to "prove" adultery so her Petition will fail. Answer "no" to Q5, and file your own UB Petition.
If you have not confessed adultery but costs are not claimed, then you should think long and hard about your reply. Really you might as well just confess and go along with it. The grounds for divorce do not affect the finances in any way so you will not be "punished" for being the guilty party. Really the cheapest and easiest way is just you go along with it.