Having taken a look at your video -

I didn't see a boom on the boat. It could just be my bad eyes but I didn't see one there. That what is causing a lot of your problems being overpowered downwind.

Second, it seems to me as if the sail wasn't fully hoisted to the top. This is adding even more bag to the sail that doesn't want to be there. If it is fully hoisted then it looked like you could add some downhaul on there.

Then you had your boards all the way down - which in heavy air can cause the boat to "trip" over itself sideways.

When I was learning how to skipper my boat - I flipped it during a round up and did a lot of damage to my boat and myself. Had to get it dragged on its side back to the beach. I can only imagine how much more difficult it is for a disabled sailing crew to manage the N20 in those beastly conditions.

Kudos!