I was in Casablanca last year and on T-Mobile's free high speed data and cellular roaming. Coverage was good, and one mall I visited had free WiFi with decent speeds. Also "Rick's Cafe" had good WiFi. It will depend on where you are, but based on those two cases, I would expect you should have occasional free WiFi in restaurants and other public places. If you are staying in a popular chain hotel, I'd assume they will have some sort of WiFi available.

Texting from the UK is probably included in your Mint plan without charge. Morocco is on most carrier roaming lists that I've seen, but you can always check with Mint in their travel notes on the website.

Edited to answer your question: WiFi calling on T-Mobile gives free calling to and from US numbers while you are roaming internationally. This also includes calling other roaming US numbers regardless of their location (Within The 215+ TM Roaming List). You can call travel companions or other US numbers even if they are not in the US. However, their incoming calls from you may not be free for their roaming plan. For instance, My wife and I use WiFi calling to communicate while we are in another country or on a cruise. Another nice feature of dual sim phones is a feature called IMS. It allows phones to roam as if it has WiFi built in so to speak. You can search this site for info on that subject.