1.) Because of the Death Star's gravity and the fact that even though it was a single fighter craft, it managed to capsize the lateral controls and auxilury power. Also, don't forget that more than just fighters were attacking the Executor. Admiral Ackbar ordered ALL ships to concentrate their fire on the Executor.
If you pay attention you'll notice that several Mon Calamari cruisers were closing in on the Executor, and while we could not see them firing upon it, no doubt they were, probably into the bridge with ion cannons (which as you know don't physically damage a ship, but weaken and shut down it's power systems) in the hope of further capsizing the larger ship, which definitely worked.
Finally, the bridge is essentially the head of the ship. Without it the rest cannot function. It was effectively decapitated with loss of power because of the destruction of the deflector shields and the incoming fire from the capital ships and fighter craft.
Despite being outmaned and outgunned the Rebel Alliance actually possessed some powerful ships in those calamari cruisers of theirs. Its duely noted even by reluctant imperial officers that the rebel cruisers are superior to the standard imperial destroyers, even though they do not have as many weapon batteries. They can last longer in a fight because of better shielding and shield regeneration.
As for the ease of the destruction, it was pure luck that there was system failure. Even without the shields the Executor could have continued fighting. What made this destruction possible however is that the agressive (albeit desperate) change of tactics caught Admiral Piett off guard and there was less time to react to multiple ships converging on his very command craft.
This wouldn't have been a problem with the Eclipse, which is just one reason of why its so damn powerful and virtually invincible. The shield generators were spread all over the ship and were themselves heavily fortified, as was the bridge and hull. Try as they might, even new republic fleets wouldn't have been able to do to the Eclipse what happened to the Executor. And with hundreds upon hundreds of weopon batteries for both capital ships and fighters, it would have destroyed any ship before it got too close to any potential weakspot. The ship was so huge and dwarfed all other ships like never before, just finding a weakspot may have been impossible and retreat was a more sound option (but even that was less likely because of the gravity well projectors on the Eclipse which prevent hyperspace escape). And even concentrated fire on one section of the ship would have been so insignifant as the firepower wouldn't have been causing enough damage and penetration of the armor and hull, and to such a small portion and area of the ship. Its like a fly biting a dragon. And this is not even considering the superlaser the ship had and that the Eclipse can hold complete wings of fighters, making for hundreds of fighters to be deployed, and just from this one ship alone.
2.) Yeah, but keep in mind with Death Star II it would have been invincible had it's shields been up without the aid of the shield generator on Endor. Let us not also forget that the Death Stars were originally outfitted to combat capital ships, and so are actually capable of doing that as well.
Seeing the obvious danger and weakness in the first design, it was greatly improved in many ways. The only way to have destroyed the second Death Star was to do it when it was still under construction.
3.) Yes, and in the case of the Rebel Alliance, they had to swing this way as they just didn't possess the funds, might, or material to create large fleets and so had to rely on starfighters in the early portions of their galactic efforts. As a result, the bulk of their strength lies in their fighters, which are vastly superior to the Empire's (at least until Tie Interceptors and Tie Defenders were being produced). Even then, the Rebels maintained the edge.
4.) I actually think Palpatine may have just been saying that in order to further fill Luke's mind with doubt, uncertainty and dispair.
"Your overconfidence is your weakness."- Luke
"Your faith in your friends is yours."- Palpatine
While I'm sure some of his best troops were there, I think it was mostly an intimidation factor. Also, to put it even more plainly, them just acting unprofessional by falling for simple Ewok tricks is a plot device to ensure victory for the good guys. Otherwise, we knew what would have happened. 😛
I understand your pet peeves though, but its interesting to comment on them.
