Surely the Fellowship took the wrong route?

[Zigûr]

Originally Posted by Inziladun

I think it was rejected because of the likelihood of Orcs being present there, as well as the idea that if the Fellowship used that pass, they would have been obliged to journey south under the watchfulness of Dol Guldur.

I think this is it. The "culling" as it were of Orcs in the Battle of Five Armies had been short lived, and I think the danger posed by Dol Guldur cannot be stressed enough. That being said, the Beornings apparently kept the High Pass clear, but that leads me onto another matter:

Originally Posted by The Mouth of Sauron

They could also have had the help of the Beornings and the Woodmen of Mirkwood

Personally I don't find this idea especially likely. The Beornings and Woodmen were trusty and hardy folk in their way I'm sure, but I don't see them having much to offer the Fellowship. As we know from the report of Glóin the Beornings charged tolls to pass through their land, so it seems to me that there's more evidence of them being a hindrance (however minor) than a help.

[IxnaY AintsaY]

Perhaps the High Pass is called "High" relative to the other option. I wonder if it may have been at a higher atltitude, and thus, maybe, even more perilous to cross than the Redhorn in the depths of winter.

We also no little (I think) about the land immediately east and south of the High Pass and west of the Anduin. Perhaps it is especially rugged, and it's certainly easy to imagine the marshy areas around the Gladden aren't a king's highway. Perhaps the region is also thick with Sauronic forces amassing for their eventual attacks on Lorien.

The land between Mirkwood and Anduin does seem passable, based mainly on Isildur's path after the Last Alliance, but obviously not without passing very close to Dol Guldur.