What if the orcs had not captured Merry & Pippin?

[Kuruharan]

Furthermore, I don't see how it would make much sense that an Aragorn-led company could even gain access to Mordor. Ranger though he might be, all paths were watched, and Aragorn (or any other sane man) would have never attempted Cirith Ungol.

Ah, yes, but consider for a moment. What is it that makes something a pass? Mainly it's just a low place between mountain peaks with reasonable access from below. There are certain gaps that become customary to use and so become "the pass." However, these are not necessarily the only ways over. Theoretically, you can go where your feet can carry you.

How does this relate to Aragorn, you ask. There might have been ways over the mountains that had no roads, paths, etc., but they might be traversable for the desperate who had no other way in. These "non-roads" would probably not be watched as carefully. Aragorn, unlike Frodo all on his lonesome, would probably be more venturesome in this type of work and have a better idea of what to look for in the mountains.

Of course, none of this may really have been possible, but it's a thought.

agree that Aragorn would not have been given the slip, and that he instead of Gollum would have led Frodo and Sam - and the whole company - as far as Henneth Anun. There Aragorn would meet Faramir.

Alas, unless Aragorn already knew where Henneth Annun was, I fear that the Fellowship might have passed through Ithilien before Faramir was there.

Consider the timeline. With Aragorn at the helm, I seriously doubt that the Fellowship would have lost three days in the Emyn Muil. They might have gotten through it on the very first day (the 27th). Faramir did not leave until the 1st. In the story Frodo was spending his first night in the Dead Marshes. If an Aragorn led Fellowship had left the Emyn Muil on the 27th, they would have been to the Morannon or beyond by then. By the time that Faramir's company arrived in the area of the ambush a few days later the Fellowship could very well have passed on to the south, or been crossing the mountains in some likely spot. However, I personally think that the Ered Lithui would have been easier to traverse than the Ephel Duath, so they may have headed east rather than south.

However, Aragorn might have known about Henneth Annun, but if he had taken them there they might have left before Faramir arrived, or if Faramir found them there they probably would have been in the greatest danger for the first moments until it could be established who they were (and this is making the very great assumption that they would be given the opportunity to make themselves known).