Ahhhhh! That could do it. Well, you know what they say about fish (and company) that have been around for more than 3 days!!!
You're right; forget Essaouira. But consider this alternative: if you leave Boumalne by bus or grand taxi in the morning you will get to Ouazazarte in time for the 2pm bus to Tata, which is a classic southern town. The run along the N12 south of the anti Atlas is a much much better route than the northern one. You will get to Tata at about 8pm (Hotel Renaissance is fine). There is then onward transport about 8 am to Bou Izakarn where you can pick up a bus or grand taxi to Tiznit.
Everyone says Tiznit is worth a stop but I never have time. Time is also your problem. As marocfan says, this itinerary is very stretched and believe me you won't get it done. One stop has to go. Anway, you can get from Tafraoute to Marrakech in a day. Skip Agadir. Take the early bus to Inezgane where you will pick up a bus to Marrakech. But the best way is to try and get from Inezgane to Aoulouz where a bus leaves at about 7am or 7.30 am for marrakech over the Tizi n Test. Spectacular.
Me? I'd leave Chefchaouen for another trip and take more time in Fes and the south. Chef. is pretty but touristy. Good walks to amazing villages from Tata (you can see them from The Mountain behind the hotel). Same from Tafraoute, beautiful village houses with not a tourist in sight.
If you want a seaside stop, try Taghazoute just north of Agadir. Beautiful beach. Last year before they develop. OMG.
