I'll address your questions one at a time. First, you can challenge it, but your likelihood of success depends on various factors, one of which is what type of timing device was involved.
Second, some MDJs will dismiss the case if the officer fails to appear. Others will call the officer and give him/her a second chance to appear if they can get in touch with them. It all depends on that particular MDJ.
Finally, if you plead not guilty and are ultimately convicted there will be additional court costs. If you are found not guilty, of course, there will be no costs.
There are other potential resolutions. I would recommend contacting an attorney in your area who offers a free consultation.
You can always fight, it just might be expensive to do so.
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Depending on the court, the non-appearance of the officer may or may not cause a dismissal. However, I wonder what makes you think that the officer will fail to appear? You will be in a much better position retaining an attorney