About the "invisible" Nazghul attacking and killing their foes... there was one little detail. While the Nazghul might be invisible, their swords are very much real. A walking sword is likely to be seen. Sure, they COULD carry along just a smaller weapon and kill one or two people... but how long until they are cornered and attacked (even if you can't see them, if enough people gather around a spot and hack away they are bound to hit some mark)
And it seems we forget that the Nazghul were very much "killable" (meaning, they can be killed). The seer said that the Witch King would not be killed by a man (but he was killed by a Woman and with a Hobbit drawing an "assist") but the other eight might have been killed. I'm not sure they'd dare to attack an army by themselves. Sure, they might kill ten people each... but eventually they will be found, cornered and ultimately killed.
With regards to the catapults, keep in mind that the Gondorian army could also fortify their positions... Dig a trench, put up a palisade... and THEN assemble the catapults. Even if the Nazghul have catapults themselves, they can get out of catapult range and then engineer stronger catapults. They have two years to do so. On the other hand, we have all agreed (I believe) that the Nazghul had limited resources, so how likely are they to be able to build new catapults and the like? or for that matter, how likely were they to have catapults at all? I think we are all discussing a siege that the Nazghul did not mean to break by force... So why bring along complicated machinery if you are going to sit and wait them out anyway?
Furthermore, about the bandits in Mordor not daring to disturb the plans of the Nazghul... I think it is a clear theme in Tolkien's work that the efforts of the bad guys were never concentrated, unless clear orders came from a higher-authority. And even then, there is a lot of dissension and discontent among the ranks. At this point in time, the only authority high enough to command ALL bandits would have been Sauron himself, who we all know was
not back in Mordor and commanding the peoples there. Therefore it is possible (if not likely) that the supply caravans would have been waylaid by bandits that fought for no other than themselves, as we see they often do.
Finally, while Minas Ithil could not re-supply until the siege was lifted, the forces from Gondor COULD and MUCH more easily than the forces from Mordor (if they could re-supply at all, which I'm not conceding since I still think they couldn't). In my opinion, if Gondor had had a mind, they would have lifted the siege for they had the numbers, better supply routes, better fighting skills and they had some experience, for even in these times the peoples of Gondor were likely to live longer than 30 years, were they not? The defeat of Angmar, with a fraction of Gondor's forces, had not happened THAT long ago at all. Therefore the question is "why didn't they do it?" rather than "why
couldn't they do it?".
So I still stand by my theory, even though your arguments are perhaps better expressed than mine. I blame it on English being a Second Language, or my science background rather than Literature
If Minas Ithil was besieged, without a large force (for we all agree that there was no large force in Mordor available at this time) and yet Gondor did not lift the siege it is because it was in some way "unconventional". To me, it'd be better explained by a haunting and guerrilla-type warfare (For example, like Faramir did before the War of the Ring) to discourage the garrison and small population at Minas Ithil than by a conventional "YOU (supplies and reinforcements) SHALL NOT PASS" siege.