Just FYI, the average amount of time a grand jury spends on each case in Harris County is about 15 minutes or less. More often than not, the prosecutor only reads the probable cause statement and gives a short presentation to them. Calling witnesses isn't the norm, it's the exception. If a DA wanted an officer's take/opinion, they would just read it from the probable cause statement.
In addition, there is nothing unusual in of itself for the defense attorney to keep in touch with the prosecuting DA on a case before it is presented to a grand jury when the defense attorney thinks it is a close case. However, I will say the amount of contact Hardin had with the prosecutor is certainly well above average. For me it's usually only a couple of emails back and forth. While certainly not routine, defense attorneys do sometimes give prosecutors a packet of information, sometimes called a grand jury packet, if the prosecutor agrees to give it to the grand jurors. The Public Defender's Office in Harris County does this a lot from what I have heard.
Of course, the DA could also re-present the case to a different grand jury so long as the alleged offenses are still within the applicable statute of limitations. More women continuing to come forward could lead Ogg to present it again, and honestly I think that is why the officers went to the media (to get more women to come forward and shame the DA). However, Kim Ogg is a little preoccupied in her feud with the County Judge.
Regardless of any potential criminal liabilities, this has been a PR nightmare for the NFL and I expect they will reign hell on Watson once they are prepared to do so. That civil trial may also not go so well for Watson, but with Buzbee saying he wants to add the Texans as a defendant as well, then that civil case could get dragged out for a while. 24 women is still 24 women at the end of the day, and it is only natural to believe that something must have happened, that all of these women cannot be liars.
In general, a party cannot introduce prior acts to attack a person's character for truthfulness. The witness themselves would have to open the door to that evidence. Their allegation by itself isn't usually enough. Though if Hardin has such evidence, I imagine he will find a way to at least attempt to get into evidence as there are plenty of questions he could ask that could get a witness to open the door to such evidence.
Texas Rule of Evidence 608(b): Specific Instances of Conduct. Except for a criminal conviction under Rule 609, a party may not inquire into or offer extrinsic evidence to prove specific instances of the witness’s conduct in order to attack or support the witness’s character for truthfulness.