Update for others' benefit: I bought Lifemiles with a 100% promo and booked 2x J return awards Sju-Pty-Hav for a net cost of approx USD1284 (or USD622 each). We returned from Cuba a week ago, and flew this return route with no problems. A few points: You cannot get the tourist card at Sju; if you haven't already obtained one prior (We obtained ours in advance for £20 each from
http://www.cubavisas.com/index.php ), you can get one at the departure gate at Pty.
On the return, it took a little while to get both the Hav-Pty, and Pty-Sju BPs. We were asked to show our etickets showing our flights leaving Sju (and the USA), and the info was typed into the computer. The computer picked up my +1s ESTA (both travelling on Australian passports) without any problems but i was asked to provide a printout of mine, and I think the authorisation code was entered manually. Took about 25 minutes all up to produce the 2 BPs each.
At Sju, we declared that we had been to Cuba, and were asked how Havana was, and had absolutely no problems returning to Sju (USA).
Oh...had a blast in Havana!^