Several commenters are eager to know what happened with the car.  The answer is that I finally succeeded in registering it . . . yesterday.  That's right, I did indeed purchase it August 3rd.

Sequence of events:

1.  Megan buys car

2.  Megan goes to get temporary plates in order to drive car back from Florida.  DC DMV informs her that she cannot get a driver's license (a necessary prelude to temporary plates) because she is a wanted woman in the Commonwealth of Pennsylvania for a 16-year-old underaged drinking offense that in no way involved a motor vehicle.

3. Commonwealth of Pennsylvania tells her that she may, at her leisure, mail in a check for the privilege of serving her three months license suspension for the underaged consumption of alcoholic beverages.  No one in the Commonwealth of Pennsylvania appears to find the prospect of a 35 year old having her license suspended for drinking underaged the least bit odd.

4.  After much begging, she is granted temporary tags by DC.  She picks up car in Florida.

5.  Attempts to rectify situation with Pennsylvania without serving a three months license suspension met with much righteous indignation from Commonwealth employees. 

6.  Car accidentally driven through red light in Logan Circle.

7.  Mother manages to pick car out of all other cars on 15th Street to swerve into and crack side view mirror.

8.  Day after mirror crackage, temporary tags expire.  Car, which now cannot legally be driven on streets, put in garage of kind sister, whose own car is too big to fit.

9.  Several months of unsatisfactory wrangling pass with Commonwealth.  Angry DMV employees stick to guns, claim that attempts to bypass system are sheer egotistical requests for favoritism.

10.  Red light ticket, her first moving violation ever, is paid.

11.  After receiving six emails from people in the same plight, Megan actually researches relevant statutes.  These seem to indicate that in fact, the Commonwealth is in violation of the law, which gives them no power to suspend the license of non-drivers in the event that they ever get one; rather, it allows them to prevent non-drivers from getting a learner's permit.  Livid, she writes a snotty letter to both the DOT and their press office, inquiring as to the reasoning behind their actions.

12.  The suspension is quickly reversed, leaving only a $25 reinstatement fee.  The reply from the staff lawyer indicates that there is no reasoning, and that everything she has been told by PennDOT employees is clearly wrong.  Rather, it simply seems to be easier to screw people and fix the illegality if the person they're screwing turns out to be important, and/or a member of the press who can broadcast their illegal actions.  (Consider them broadcast).

13.  Fine is paid.  To be sure, it adds insult to injury, but it's not worth fighting about.

14. Car is driven to the DMV, where she is informed that she cannot register it because New York now has a hold on her license.

15.  Inquiry reveals that the hold is related to the Logan Circle ticket.  Presumably, the check was received late and the fine doubled, causing the DC DMV to reject the check.  Ticket is paid; proof is faxed to New York.  Car waits in sister's garage for another five days while notice clears their system.

16.  Thanksgiving

17.  Car flunks inspection because of cracked mirror. 

18.  Megan begs DC DMV employees to give her another set of temporary tags so she can drive car to nearest dealership in Sterling, VA (near Dulles) to fix it.  DMV employees tell her that head office believed that local DMV managers were being too promiscuous with tags, and have now set up computer to reject any and all requests for a second set of temporary tags.

19.  Dealership is called.  Next available appointment is after New Year's.

20.  For $30, a glue on mirror is ordered from the Internet.

21.  Christmas

22.  Glue-on miror arrives.

23.  Moving.

24.  New Year's.

25.  Glue-on mirror attempted and found too small.

26.  Car taken to dealership.  Mirror repaired.  Car driven to DMV, where it is discovered that half the supporting documentation is out of date.

27.  Running around city getting copies of new lease, etc.

28.  At 2:30, Megan emerges triumphantly from DC DMV, car registered a scant five months after initial purchase.  Comforts self with thought of all the equity she has built up, making payments while it sat unused in sister's garage.

Update:  To those somehow convinced that this is all my fault because I let my temporary tags expire/ran a red light/ordered the wrong sized mirror, let me clarify.  The reason my tags expired is that temporary tags in the district run for one month, and the Commonwealth of Pennsylvania refused, during that time, to release the hold on my driver's license.  In the District of Columbia it is not--or so I was repeatedly told--possible to register a car with an out-of-district license; and due to federal regulations, they could not issue me a license while Pennsylvania still had a hold.

The red light I plead guilty to, but that particular light is a longstanding problem that DC has so far failed to rectify--it's badly placed, and consequently a lot of motorists, including me, miss it.  The failure to fix the problem suggests to some that this is less an attempt to manage traffic than an attempt to manage revenue.

As for the mirror, I ordered the mirror specified for the make and model of my car.  I'm not sure how a vendor supplying the wrong mirror is something I could have fixed.