Time stop ends if you "affect" another creature by your actions.
So, in order:
Counterspell, no. That would have to be done before the spell takes effect.
Hellish rebuke, yes. It happens after the damage has been rolled and applied.
"Try to reduce the damage", depends.
Consider shield. As written, it isn't really functional, because it can only be cast after you've been hit, but then can make the attack not hit you, which would imply that it couldn't have been cast.
The sensible interpretation is that when shield says you're "hit" by the attack, it means that the attack has been rolled, and would hit you, but the attack is not yet a hit; there's still time for some kind of very fast trick that will save you, such as shield. And then the attack causes no damage because you never actually got hit.
But if you're being attacked by someone who's cast time stop, then their actions have not affected you until you actually do get hit. If shield were to make the attack miss, then you'd still be frozen in time, and therefore you couldn't have cast shield. So that doesn't work.
On the other hand, absorb elements has to be cast after you take damage, which suggests it may work. That's right on the edge though; I could see a DM ruling that since it reduces damage from present and future attacks but not from the past, it logically must be cast before the damage happens.