To OP:

1. This being FT, where detours bringing home more TP/miles/points/shiny cards etc is the SOP. $300 for 280TP is a good price, especially when comparing the price for "only" 280 on the direct flight. You need the TP? Go for them.

2. Shopping in New York is a plausible excuse, understandable even for your friends, who never think outside the box of direct flights. There are few opportunities of quality shopping mid-Atlantic IMHO. Assuming you are not only shopping for yourself, but for someone you care for, this is what to emphasize

3. The Flagship Lounge experience at JFK might not be the most important thing to consider in chosing here, just see it like a nice bennie as you've been good to yourself (1) and someone you care for (2)

Enjoy your trip, especially the fruitful routing through NYC