Just to confirm, then:
You apply the "half speed" due to being crippled, then the speed modifier from engine damage. So if a ship with normal speed 8 is crippled and has taken a -2 Speed critical, its speed is now 8/2 - 2 = 2, not (8 - 2)/2 = 3. Being reduced to speed 2, it can move 1", then turn.
And if it's taken the -4 Speed critical, its movement is 0, it can't turn normally, but it can All Stop And Pivot. (Bryan does not appear to have contradicted that specific point by Burger during that thread.)
How much of the above is correct?