In the nearby Canary islands coconut trees grow well in a just slightly warmer climate. They are being planted everywhere along the beaches. Fruit are not uncommon but when i picked them i got about 30% good nuts with flesh and water, 60% looking normal on the outside but with a espindle shaped shell, a bit of flesh and no water. Some 20% (but usually 100% from one tree look very normal o the outside, up to 1.5 kg, plump and green but when you cut them open there is no shell, no flesh, no water, just fibers.
Maybe the climate is not perfect but you can get good coconuts there. My point being do not give up, i am sure with increased coconut cultivation for landscaping in Madeira you will get your nuts sooner or later😉