Armour: For a comparison of these two, we must look to a common baseline, some attack which exists in both sources: Asteroids. The GTVA's destroyers are shown able to withstand impacts by several asteroids. An Imperial Star Destroyer's Bridge tower is struck by an asteroid of similar size, though greater speed, and is destroyed. This suggests that the hull armour of the GTVA's ships is greater than that of a Star Destroyer, though debatably considering the Asteroid seen in Star Wars had a greater kinetic energy.
We don't know that the bridge in ESB was destroyed. All we saw was its commander getting knocked around and his transmission cutting off. It could easily have just shook the bridge and damaged their comm system.
And at any rate, the fleet had been in the asteroid field searching for the Millennium Falcon for quite some time when that happened, much longer than a GTVA ship would be able to survive in an asteroid field. Those asteroid missions are over within minutes and the ship's hull is still typically at critical levels by the end of them. I agree with the rest of your points, but this scene in now way indicates that Imperial armour is weaker than what the GTVA uses, and considering the fact that it's able to stand up to barrages from the much stronger Star Wars weapons I think it's safe to conclude that Imperial armour is significantly stronger than GTVA armour as well.
I just had another thought, some people say that the empire has hordes of fighters and bombers (no doubt true), but do any of those fighters have shields? Do they have weapons that are actually useful against FS shields (xazer, etc)? This would actually give FS a huge advantage, especially flak equiped capital ships (any unshielded fighter can be torn apart in seconds by flak, regardless of the universe).
All I know is that I want to see a GTVA vs Empire campaign now. How cool would it be to capture a Tie Defender 'Playing Judas' style, or do bombing runs on a Star Destroyer's bridge, or try dogfighting with Baron Fel?
Sounds awesome, so who's going to start fredding it? 
Shield interactions are visible on TIE-Fighters in the movies. It's only the EU that says they're unshielded, and the EU also adds quite a few shielded fighters to the Empire's arsenal. Also, we know that the Rebels are able to shield their ships, so it's obvious that the technology exists to do so in the Star Wars universe, even if the Empire may choose not to employ it universally.
thus rendering Fighter attack relatively useless against the might of the Imperial shielding
Then how did A-wings destroy the shield generators in ROTJ in about five shots?
The entire Rebel fleet had been bombarding the
Executor with turbolaser fire for quite a while before the A-Wings swooped in, took advantage of the ship's dropped bridge shields and destroyed one of its sensor domes. It was the fleet that took down its shields, not the A-Wings.