The DMV is clearly dragging its heels and your filing a writ of mandate may force their hand. There is no reason for the delay you describe when you have been acquitted in Court of the DUI charge.
Because the DUI charge was either never filed or dismissed in exchange for a plea to drunk in public, you are entitled to a new hearing by filing a DS-702. This form requires the DA to state the reasons why DUI charges were either never filed or subsequently dismissed. Upon forwarding the completed form to the DMV, you will be afforded a new hearing, where you can introduce into evidence the fact that DUI charges were either never filed or dismissed. I do not see any reason why this form cannot be filed with the DMV while Departmental Review is still pending.
A writ of mandate is a complicated and costly procedure, which does not arise until after the Departmental Review process is completed. You could have gone straight to a writ, but you opted for Departmental Review, which means, you need to wait for that process to end for you to be able to file the writ of mandate. This is why the above option is likely better: not only is it less costly and complicated, it would likely avoid the waiting game you are currently experiencing. If you file a DS-702 and provide it to the Departmental Reviewer, you may stand a greater chance of a more favorable decision from that person as they will know that, even if they uphold the hearing officer's decision, you are still entitled to a new hearing.
You may want to contact an attorney who specializes in these types of issues with the DMV as you likely do not possess the requisite knowledge of the inner-workings of the DMV and one false move could result in your continued aggravation. Sometimes it is worthwhile to spend a little bit of money on the front-end for a favorable result on the back-end.
Note: This assessment is provided based on the limited information provided.
If you were represented by an attorney in court, I would consult with them regarding your next step. Certainly a writ can be filed to force the DMV's hand into resoliving this issue. There are also other things that can be done, through the court, to reverse the DMV decision. If you were not represented, I would suggest contacting a DUI specialist member in your area. California DUI Lawyers Association has a number of local, well qualifed members to assist you.
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