В тёплое время года да.

The airport train departs from Gare Casa Port railway station every hour on the hour from 5am to 10pm.and takes about 45 minutes. It also stops at Gare Casa Voyageurs enroute (about 13 minutes after departing Casa Port). If you're staying in the Ain Diab area, it would be possible to take the Tramway from Ain Diab to Casa Voyageurs and join the airport train from there. The problem you need to address is the timing. My feeling is that the Tramway is perhaps going to take some 45 - 60 minutes just to get you to Casa Voyageurs, and you can't afford to miss the intended train because the next one is one hour later. Secondly, on arrival at the airport, there is a terrible crush of people (from the whole airport train) wanting to enter the airport, and that doorway has only one metal detector and one bag x-ray machine. My observation was that it took some 45 minutes to clear the line. In other words, unlike most airports where you can comfortably arrive 2 hours before flight departure, at Casa airport, you need to reach the metal detector at least 2.5 or 3 hours before departure. Add that to your train timings and you may have to leave your hotel real early.

Probably by Uber, might be the easiest. You can take a shared white cab out or pay to have one all to yourself. If the trains are running take a red cab over to the train station and ride the train out. Watch your timing though as the trains can be unreliable.

NO, it's not on the way form the airport, but you can catch a cab to the beach from anywhere in the city, or have your driver take you straight there. Ain Diab is also NOT close to the ONCF station at the Port. Again, a red cab can get you there in a few minutes, if traffic isn't bad. The tram does have an Ain Diab station if you take the Tram in the city, but a cab is probably the quickest/easiest.