I, actually, lived on LI around the time that McCarthy was forced into politics and I remember the story well. But I am not sure, how easy it would be to play off her ancient Republicanism. She was a fairly apolitical woman, cast into politics after the murder of her husband by a maniac on LIRR. You bring up her origins as a Dem, you also bring up the memory of that story, which would remind people why she was elected in the first place. She has been a consistent suburban Democrat ever since. Yes, she's been a few terms less in Congress than Ackerman (16 vs. 29 years), but it is not negligible seniority as it is. She may be only a couple of years younger, but women live longer: her life expectancy should be taken into account when thinking seniority.
The North Hempstead thing is more serious, though. But Ackerman is an NYC, Queens guy; McCarthy is the proper Long Islander. True, North Shore is North Shore, but is there any reason to believe Ackerman is a better overall fit there than McCarthy? I would want to see the map first.