Join Date: May 2012
Location: MCO
Programs: AA, B6, DL, EK, EY, QR, SQ, UA, Amex Plat, Marriott Tit, HHonors Gold
Posts: 12,810
In my experience, yes you will be able to use the luggage allowance of the highest class of service segment in that particular outbound set of segments.
You'll definitely be able to access the SC in ATL. I'm not sure about JFK however. My guess would be no unless it somewhere states that you can access any SC systemwide on the same day you hold a BizE ticket.