A: If your lawyer talks to the witness, then your lawyer becomes a potential witness.
Here is how: Witness tells your lawyer that the meth was actually theirs and that you didn't even know it was there. Then later while in trial, the witness gets cold feet and says that they don't know anything about any meth. Now your lawyer is a witness to tell the jury that they are changing their story. This is called impeachment. The reason an investigator is used is so that there is a different person that the lawyer can call to the stand in this situation and ask what the witness said before.
Additionally, your attorney does not want to be accused to telling the witness what to say. If there is someone else interviewing the witness, then that won't happen.
A: Lawyer up. What you are wanting to do may not have the affect that you think it will have. Rather than dismissing the charge on the person you are trying to protect, the DA may just charge you both if you now say it was yours.