According to a criminal defense attorney @ this website:
http://www.avvo.com/legal-answers/what-happens-when-a-witness-is-impeached--1585162.html
No one declares a witness impeached. That is up to the fact finder (jury or Judge if a bench trial). The fact finder is generally allowed to accept the part of the testimony they believe and disregard the rest or disregard everything. This is called the falsus en uno charge. Just because a witness is "impeached" does not mean the witness cannot testify and does not result in automatic dismissal.
The fact finder in this trial is Judge Masipa. When the trial has concluded and she deliberates, she'll decide whether or not a witness was credible, whether or not to accept part, all, or none of any witness's testimony.
Roux is clearly trying to discredit (impeach) all of the witnesses' testimonies. He's tried to do so with each and every ear witness, the State's witness from the Silverwoods estate, each and every witness related to the firearms charges, and every forensic witness (with the possible exception of the ME, but there's no way to know for certain because Dr. Saayman's testimony wasn't broadcast).
Roux has employed these categories of impeachment:
1. Bias
2. Inconsistent statement
3. Contradiction
He's even gone so far as to accuse Van Staden (crime scene photographer) of lying.
I will concede that there have been some concerning incidents which occurred during the investigation (the allegedly 'missing' watch(es), the preservation/storage of the toilet door, the uncollected splintered pieces of the toilet door).
I have found the ear witnesses to be credible in their recollections. I found Pieter Baba to be credible. I found S. Taylor & D. Fresco to be credible, as well as Sean Rens (Roux couldn't get him out of the witness box fast enough, IMO). Aside from Vermeulen, I find the forensic specialists who've testified thus far to be credible.
This trial is in its 2nd week now. It's become clear (to me) that Roux's defense tactic to try to discredit all the witnesses is because he knows the law is not in favor of his client. At the very least, OP is guilty of culpable homicide according to SA law. I think Roux is hoping to get the case dismissed if he can sway the Judge to believe the investigation was flawed and the witnesses aren't credible.
I think Judge Masipa is smart enough to see through his tactics. I think she will weigh the witnesses' testimony and the evidence. I also think she possesses the wisdom and experience to know what to accept and what to disregard.