The only way I have found to avoid his dance scene is to have both act 1 "romance slots" filled before entering the shadow cursed lands, otherwise he just gives it a shot - no matter your approval or what you talked about before. Personally, I think the scene only exists to make Astarion jealous and manipulate him into admitting that you are more than casual bedmates.
As for Shadowheart, I ignored the "we should get to know each other better" line for all of act 1, so she asked me out for a bottle of wine during the party and disapproved of not getting kissed in the morning. I thought, we were just having a friendly chat, Shart? ^^
