I'm lvl 34 and with several years worth of experience with 1h/shield (never having played 2h). I'm pretty poor at weapon blocking; whenever I go to block an overhead, I accidentally move the camera and obstruct my view of the enemy and die, so I use a shield so I don't have to deal with this (also ranged are annoying).
I spent 2 hours on the duel server with a Skip The Fun 2h character (21/18 or something) with a +3 Longsword, and I was routinely beating people that I can't beat with my main (mostly agi polearmers, kick spammers, etc). And this was only bothering to block sideswings and thrusts (and just trying to dodge overheads), and I don't even know how to execute hiltslashes or any other "advanced" 2h techniques. I just ran around erratically swinging and stabbing a lot; the huge 2h animation sweetspots took care of the rest.
The moral of the story is that if you can't be effective with a +3 Longsword, then you are probably over-thinking it; just move around randomly and swing. I have honestly never seen anyone that uses a +3 Longsword/HBS perform poorly.
Phew, I wish you'd stop this crusade...
1 - 2h generally is what people use to duel with in many situations because 2h has good animations, and a balance of speed, reach, and damage. You are correct that in a duel situation a 2h is probably better than your 1h/shield, whereas you are also right that in siege the opposite can be true due to small spaces.
2 - I somehow don't believe you that the people you were beating are people you usually do not beat, since you do rather well in siege. Who were these people? I agree that a change of weapon class could help if 1h/shield has a weakness vs these players in their builds, but I am not fond of the idea that you seem to feel any idiot can pick up a 2h sword and do well.
I for one, have seen many players with LS/HBS do poorly, but you remember the ones that do well, not the ones who do not...that is something everyone in any balance discussion should remember.
"moving around randomly and swinging" with a longsword may help you take out a couple unknowing opponents or noobs, but I assure you that it will not result in success against anyone decent.