Legal Question in Criminal Law in California

Re: testimonies

By bus, auto, or taxi.

It's up to the defendant's lawyer to bring out the reasons why the witness' testimony should not be trusted. This is why it makes a big difference which lawyer is defending the case. The public defender won't be able to prepare for cross-examination of trial witnesses nearly as well as a private lawyer. Sometimes the public defender doesn't prepare for trial at all, and simply tells the defendant the case is lost and he must take the DA's offer -- in which case the defendant usually has little choice but to plead guilty. Some privately retained lawyers do this as well, so make sure the defendant in your case has a lawyer who is well prepared to go to trial.

Re: testimonies

No one is barred from testifying. The factors you describe may affect the witness's credibility, but they won't keep him off the stand. If the jury learns of reasons why the witness might be lying, it can take those reasons into account when deciding whether to believe him. Giving such information to the jury is called impeaching the witness.

Note that defense counsel may not even be able to raise some of these issues in front of the jury. A "pending felony case", for example, is not the same thing as a felony conviction. A pending case means only that someone has made an accusation, not that the accusation is true. The court likely won't allow defense counsel to use the pending case to impeach the witness.