Shouldn't be any real issues getting into Boston at that hour - even with traffic on I-93, it should take no more than 45 minutes.
Even going on the back roads via North Andover and Boxford to I-95 would take only 45 minutes from Lawrence (not that I'd recommend it for someone that doesn't know the area).
Is a one-way rental from BOS to Lawrence possible, and then take the train back into the city?