1) I would try to summon something of import to him, either something he's lost and couldn't find again, or something he couldn't get himself.
2) Before 'future knowledge', my response was always 'Hell no! Leave the littel moo alone!'. With future knowledge, however, my response would be to apologise to Asterion, but inform him that by doing this, we get the mirror, and he doesn't have to go out again.
3a) If there was one thing I could tell Asterion, it would be simply this. 'I do so swear, by blood, by soul, and by my very flesh, that I will not knowingly, willingly, or actively seek malicious actions against you and all you hold dear.'
3b) If there was one thing I could 'do' for Asterion, it would be to try and give him some form of freedom, while keeping him, his wishes, and his dreams in mind. Maybe a way to safeguard his wine, or have a personal 'pool' of wine, similar to a medieval 'bath', that he could access at any point in time. Maybe a freedom from orders that last more than 24hrs without further input from his master(To prevent what we walked in on in the first place). Perhaps a way to 'cancel' his current master if they go beyond a certain point? Regardless, so long as it was for Asterion's benefit, I could do a fair few things.