I don’t think parking it up and buying another car is the correct route to go.
By all means talk to Motability and tell them that you have grave concerns over the actions that one or more dealers have (or haven’t) taken. Express to them clearly that you are willing to let the dealer/ Hyundai rectify the problems properly but that in the event that they are unable to do so that you are unwilling to continue with a vehicle that is not performing within its expected parameters.
I don’t believe that if it’s not properly rectified that Motability will force you to continue with it, but you do realistically need to give them chance to rectify the situation first.