as for the shield generator dish explosion - the dish was a long way off, they planted charges at one end of the power core tunnel and the explosion snowballed as it travelled to the dish (just how i see it mind you, i have no data to back this up)
i'm pretty sure the reason a short burst of lightning killed vader is because it destroyed his breathing equipment
there have been some great muppet moments over the years but they DO NOT belong in star wars, any de-muppeting is probably the most important change needed to rotj