How can we interrupt the Witherbloom Apprentice and Chain of Smog combo? (STX spoilers)
You can interrupt this combo with any instant or ability that removes the Witherbloom Apprentice or counters the Chain of Smog.
The most important thing to see here is that the copy of Chain of Smog is created as part of the resolution of the spell. The player it's targeting can't create the copy whenever they want, while the spell is on the stack. The fact that it's a sorcery is irrelevant. So, Pongify will work, because its an instant that removes the Witherbloom Apprentice.
On the other hand, Split Second on Krosan Grip is irrelevant here. Sure, while it's on the stack, the opponent won't be able to do anything else, but after it resolves, the Chain of Smog is still on the stack, and the combo still works.