I am not a traffic lawyer, but, here's my take on this:
First, it's quite likely that no radar detector would have provided you with an adequate warning in this particular circumstance.
Pennsylvania state troopers running instant-on K-band from a stationery position can be quite lethal (especially at night) as they wait until you are either very close or occasionally target from behind (from over any overpass) before pulling the trigger. Being the only one in the area meant you were a sitting duck and had no other proximate vehicle ahead of you to provide with an advanced radar warning.
Being, a PA resident, myself, I am all too familiar with both RADAR and VASCAR speed enforcement and I too recently got cited for an obstruction of windshield during a recent traffic stop (in NJ, actually) for having a Valentine 1
mounted low on the center of my windshield (along with couple of citations)--the young officer had a "John Wayne" syndrome, as well (which is common with newbies).
Now to your questions.
I would completely detach yourself emotionally from this incident, request a court hearing by pleading not guilty, and then make following request at your hearing.
1) Dismissal of the obstruction charge, and
2) Acceptance of a guilty plea to a PA-3111a (for your 71 in a 55 zone).
If you are fortunate, both of these requests will be granted and the PA-3111a ("failure to obey a traffic controlling device") carries no points in PA, costs a little more than $100.00 with court charges). You could also offer a plea of guilty to 5 over (60 in a 55 zone) which again doesn't carry any points.
I would not challenge on the basis of operating police radar with no lights. They do this routinely and sometimes from a hidden position from the rear on an overpass.
Make these requests without emotion and without explanation or argument and hope you get a break by either the judge and/or the officer.
You have something in your favor, in my opinion. 71 in a 55 zone is not too bad and your traffic conditions were conducive to this.
It's important to be entirely detached from the emotional upset of this incident, acting as if you were the attorney making the request for your client.
If you are especially fortunate, either the judge or the officer (if he shows up to court) will make the overture with an offer. If it's not both of these, I would counter with them and hope your request is accepted.
Good luck, with it, and please provide a follow-up post when this is all said and done.