I just found out, that you now can basically tell Astarion to get lost during the first conversation - that is a huge plus. Again, his really standoffish introduction should be punishable in some way - this is a start.
you can avoid the wrestling in the ground without a check? That would change a lot for me. Unless they let me avoid that without a roll or at least let me attack him through dialogue, he'll never be on my team.
No, it's the discussion afterwards. You can basically tell him, that he has to go on alone, impliying, you don't want him in your group. Maybe you meet him again later, but he isn't in the druid grove, so maybe he joined the goblins.
"We are all stories in the end. Just make it a good one."