It is generally advised to leave at least 3 hours to make an international transfer. They will, as you correctly assumed have to see a customs official, not a kiosk. Personally, I would probably take the 1558 flight.

You indicate they are on two separate tickets. They are not transferring anything, so they will need to take their bags and check in. The transfer desks are for people on one ticket with a change of aircraft in the US. Delta leaves from both terminal 2 and terminal 4. If they are lucky, it will be T4. They would have to take their luggage upstairs, check it in and then go through TSA security. I think ( maybe go to the Delta forum and verify) that even if they are flying out of T2 they could theoretically check their bag in at T4 and then take a jitney shuttle ( Delta only) AFTER security from T4 to T2. Personally, if they are going out of T2, I'd bring my luggage there. --less chance of it going on its own journey!

They do not need to drag bags onto air train. If they rent a luggage cart, they can take the cart to the air train. (use elevator) and take the cart ON the air train to T2.