But at least Lucas tried to explain why the Empire does nothing. They are arrogant asses who don't really see them as a threat. Tie Fighters were also shieldless crap. Incredibly cheap to build but quick and effective in numbers.
The Y wings were defenseless because their shields were on double front to soak up the AA fire. Another part explained. For those that haven't played X-Wing...double front means all the shields are to the front and there is nothing behind you.
The lack of escort is part due to the fact that they don't have room to really move much in that trench. Besides that the rest of the units were busy taking on the rest of the Ties. Vader of course exploits this and takes advantage of it. Notice he really takes his sweet time. He doesn't come in guns blazing all over the place. He takes his time to line up his shots because he knows he can.
Why don't they evade him? Simply because at that point it is do or die. Breaking away and leaving the trench means they might not get another shot. Remember Vader is a famed pilot too. Its a checkmate situation. What can the other ships really do? Pull away? No it wouldn't help. Vader would stay on the leader.
As far as the Death Star goes, again a hint is dropped that they really don't consider the rebels a threat or more likely, small ships a threat. Again it's arrogance but it is either hinted well or explained. At the most the rebellion has one frigate at the time of Episode IV and a lot of transports or corvettes. If you play the X-Wing game this becomes very clear. Heck the Death Star missions were the only ones I didn't finish. Freaken things were insanely tough. You just have no real support and everything is a race against time.
Another factor to consider, but this is only really revealed in the games and later lore, is that the Imperial fighters were not ultra seasoned. They put down rebellions or just waste people here and there but they don't really have any major foe to fight. Stormtrooper accuracy is a good joke on this. The rebellion fighters are from day one put into the fight. They get seasoned much quicker.
This somewhat goes back to the Clone Wars too. Clones beat droids for similar reasons. Specially the dumb droids of the Feds. They learn, adapt, and get that seasoning.
So yeah, IV had issues but just those ones there are ones that are either hinted or explained. They can also be inferred rather easily. But here with JJ you've got big plot holes being left unanswered. This article mentions some of the big ones:
http://io9.gizmodo.com/everything-thats-wrong-with-star-wars-the-force-awaken-1751756919Would Lucas have left the same major holes? Maybe? I dunno. I remember some of the cut scenes of the prequels being moments where I thought, "why the hell did you cut that out?!" "It would have made so much better sense." But overall I think at times he tried to go into too much detail and ended up boring the fans.