And on the other side of the fence, I feel like myself and others have stated dozens of times that the film itself does suggest a few possible ways he could have gotten in. It may not actually tell us, but it plants a few seeds that can grow in your imagination.
I also don't think the film made a huge deal of effort to show that it was so hard to get into the city. All we're shown is the army being forced to guard the one remaining bridge (and supply trucks are being allowed through). It's a big island. It'd be different if there had been a scene that showed a mercenary sniper picking off some guy who was trying to sneak through on the ice. But they never show that, because Bane/Talia weren't concerned about that. Their main goal was to keep the US army (or its allies) from interfering. The whole point of that scene with Barsad and the army on the bridge is to establish that, no, they don't have the manpower to guard the entire island, but they're gonna force the army to either call their bluff or not. And the emphasis is on people trying to get off the island ("Why would we help you keep your hostages?"). If the movie was trying so hard to show that it was hard to get on the island, they should have cut the lines about setting up "terms of access for supplies and communication" and the whole special forces subplot. That to me painted a different picture than the one you're describing.
And I still say as a plot omission, it's no different than the way Bruce is able to put together the unprecedented sonar machine completely off-screen in TDK in such a short amount of time, in such a way that NOBODY (not Lucius Fox, not the government) knew what he was doing. Then again, maybe you can get into a debate about whether or not he was technically the main protagonist in TDK...
Anyway, I get it. It's just a fundamental difference of opinion, as are most of these TDKR things. I'm only trying to clarify my point of view.